When Sherlock arrived at the sea floor, Mycroft was already swimming towards the castle, pushing the doors open in rage and darting to his throne, so as to look more intimidating. When Sherlock finally arrived (he was taking his dear old time, trying to appreciate having his own free will while he still could) Victor was floating shamefully in front of Mycroft's throne, muttering last minute apologies in hopes to soften his sentence a little bit more. Sherlock arrived next to Victor and crossed his arms, trying to look as inconvenienced as possible, which really wasn't hard. The hall was empty except for a couple of late night guards, standing watch at the doors and looking extremely tired. Sherlock looked around for a moment, at the large coral pillars, the plush red carpet, the smooth walls speckled with pearls, it really was gorgeous, but he'd appreciate it a lot more if he knew it wasn't his execution chamber.
"Words don't even explain how upset I am with both of you." Mycroft started.
"Good, then don't talk. Can we go?" Sherlock muttered with boredom. Victor gasped, obviously not used to Sherlock's shameless sass.
"Sherlock if you so much as swim one inch I will throw you in the dungeons." Mycroft warned. Sherlock groaned, but didn't move, knowing that his brother would be true to his word.
"My lord, we really are sorry, I promise, it won't happen again." Victor muttered, looking about ready to kiss the hem of Mycroft's robes just to get himself out of trouble. Mycroft sighed, obviously unamused, his fat tail swishing in annoyance.
"You're right. It won't. I'm demoting you, Mr. Trevor, to guard duty. I will find Sherlock another bodyguard and you will not be seeing each other again." Mycroft decided. Victor made a small noise of sadness but nodded, bowing as deeply as he could. Sherlock groaned, great, how was he supposed to seduce another body guard to do his bidding?
"And for you Sherlock, you are grounded." Mycroft decided.
"What are you going to do, take away my books?" Sherlock snapped with amusement. The trouble with grounding someone who did nothing already was that there was nothing to take away.
"You are grounded, and you will stay in your room without books or paper or pencils or anything you like to do in your free time, so you can stay there for a nice long while and think about what you've done." Mycroft decided. Sherlock couldn't help but gape, and upon seeing his brother's anger, Mycroft smiled his annoying little smile.
"Mycroft we need to stop John from marrying someone else!" Sherlock exclaimed. Mycroft just laughed, shaking his head.
"Sherlock get your mind away from that pathetic human for once and think about yourself. There's nothing you can do to stop his heart from wandering because after all, what does it need cling to? Now get out of my sight, both of you, Mr. Trevor I expect you at the armory at six o'clock tomorrow morning to get ready for your first assignment. I now have to go invest in a new body guard at two o'clock in the morning." Mycroft sighed. Sherlock growled in anger but couldn't do anything but turn and swim angrily out of the throne room, not with a word of thanks to Victor, and locked his bedroom door. Great, just great. Now he'll never have another chance to see his John, now he'll never stop John from sailing away, now he was stuck in his bloody room for all of eternity! Stupid, so stupid, how could he have been so careless? Of course Mycroft would find out, he always found out, and now Sherlock's life was ruined and Victor's career went down the drain and John was going to get married. Sherlock took a deep breath, slinking back against his door and sliding defeated to the floor. John was going to sail away and forget him forever. Sherlock cradled his face in his hands, letting even more salty tears roll down his cheeks and get carried away by the ocean. He had lost all chances of him ever getting to see John, he was never going to fall in love again, he was never going to kiss someone again, he was never going to be happy ever again. Because John was the only person that made Sherlock happy, and now he was gone for good.

When Sherlock woke up, he had an odd feeling he wasn't alone. He snuggled deeper into his kelp blanket and tried to ignore the fact that he felt someone watching him, surely it was just his imagination? But no, when he finally opened his eyes and looked around, he saw another pair of eyes looking back. Sherlock yelped, pulling his blanket over his face in case this new merman was here to hurt him. Instead, he just stared, his iron grey tail flicking through the water. He was extremely muscular, with solid arms and six pack abs, dusty blonde hair, not unlike John's, cut into a crew cut on top of his head.
"Hello..." Sherlock muttered, not entirely sure what he was supposed to say in this situation.
"I'm your new body guard, Sebastian Moran, Mycroft told me you are not to leave this room." he said firmly. Sherlock groaned, sitting up and cracking his neck a little bit.
"You're my new spy, huh? You seem a lot more...unpersuadable than the last." Sherlock decided.
"Yes sir, I do what my king tells me, nothing more." He said proudly. Sherlock sighed, nodding.
"Ya, that's what I was worried about." He muttered. Moran looked a lot straighter than Victor as well.
"We had breakfast taken in for you, and while you slept they took away all of your books." He said very firmly, sounding like a prerecorded message on ananswering machine. Sherlock looked over to see a cart with a little bit of food on it, obviously Mycroft wasn't feeling very generous this morning.
"So basically your job is to make sure I have as little fun as possible?" Sherlock clarified. Moran nodded stiffly, his posture not breaking once.
"Yes sir." He agreed. Sherlock sighed heavily, falling back into his pillows and deciding that he could deal with this later.
"Brilliant." He muttered. Sherlock spent the whole day doing absolutely nothing, and even though it sounds glamourous and peaceful it might have been the most horrible day he had ever had. He had nothing to read or draw or write, he had nothing to look at or play, he just lay in his bed, knowing that Moran was watching what little moves he made. He made no attempt to make conversation, Moran didn't seem like much of a chatter box, so he just lay in bed, helpless and defeated, wondering what was going on at the shore above. John might be getting on his ship now; he might be loading up his possessions and starting off on his journey across the seas to get Ms. Mary Morstan, fully prepared to leave Sherlock behind. It was a pain worse than death to be honest, to know that the person you love was fully prepared to love someone else, and to be able to do nothing about it. No matter how much Sherlock moped and moaned he knew that there was no way he could ever stop John from leaving him. Sherlock watched from his window as the boat swayed and rocked in the water, the shadow looming over the town and his happiness. He knew that when it did leave, he would have to watch it go, and know that John was on that ship, looking across the ocean in wonder and excitement while Sherlock sat at the bottom of the ocean and wished he could drown himself.
"What are you watching for?" Moran asked on the third day of solitude. They never talked, not because Sherlock didn't like him, which was his excuse for Victor, but because Moran didn't seem like much of a chatty guy. And he didn't respect Sherlock's privacy as much as Victor had. He sat shamelessly in the middle of the floor and watched as Sherlock stared out the window or lay in his bed, bored beyond compare. Mycroft was such a horrible merman, he really was. Just because Sherlock was chasing his dreams, Mycroft decided that he needed to be punished and kept in solitary confinement with the most boring, dull merman under the sea.
"I'm not watching for anything." Sherlock lied, slouching away to dive into his bed, quite literally.
"Mycroft said that you'd be watching for someone, a human on that boat." Moran pointed out. Sherlock sighed; he really didn't like conversations with this one. He seemed intelligent, like he was talking to find more out about his prisoner rather than talking because he liked to hear his own voice. Victor had been chatty about just about anything, except Sherlock rarely answered. 
"Surely you've heard the stories about what I've done." Sherlock sighed. He Wasn't looking at Moran, but he could hear the merman's confused silence.
"I haven't heard any stories about you." He pointed out. "Only that you got your last body guard fired."
"Good. Then you're one of the few under this ocean that doesn't think I'm insane." Sherlock sighed. There was another silence, and he knew that Moran was expecting him to say more, because the suspense was obviously killing him. Sherlock didn't really want to tell Moran about what exactly happened with John, but then again, maybe he could guilt trip him to join his side, like he had managed to do with Victor. Maybe Moran would help him stop the boat once and for all.
"What did you do?" Moran muttered, as if he felt bad for asking so directly. Sherlock sighed heavily, as if it were such a burden to tell a single story.
"I took a potion to become a human." Sherlock muttered. Moran gasped; as if that was the most scandalous thing he had ever heard. Sherlock just rolled over on his bed and stared at the wall, so that Moran couldn't see him processing how to make this story sound as emotionally taxing as possible.
"You mean you went up to land? But why, why would you have such a ridiculous idea?" Moran asked.
"I went up because I fell in love with someone I saw on the docks, someone I had watched before I knew it was wrong, someone that is leaving on that boat any day now." Sherlock admitted with a deep sigh, his brain wandering to think of John on the top of the ship, looking out over the open seas in a ridiculous captain hat. He still looked beautiful.
"Why don't they come back for you?" Moran wondered. Sherlock tried not to get too worked up at that question, the one he wished he could answer even though he knew the truth.
"Mycroft found out. He kidnapped him and made me give him a potion to erase his memory, and now he has no idea I exist." Sherlock sighed.
"Mycroft knows you exist..." Moran started.
"Not Mycroft, John, you idiot! John doesn't remember me!" Sherlock snapped, chucking one of his many pillows in the general direction of Moran's voice for good measure. There was a bit of a silence, and Sherlock was getting rather concerned that he had somehow managed to kill Moran with the aerial pillow before he heard a little bit of a sigh.
"I'm sorry to hear that." Moran muttered. Sherlock was expecting more sympathy, at least a little bit more than 'I'm sorry to hear that', but then again, Moran was a very uneventful merman. His heart was probably as steal as the color of his tail. So Sherlock lay more in his bed, thinking up any possible way to get to John, however impossible it was. He knew that there had to be a potions room in the castle, even if he didn't know where. The day Mycroft had captured John, he had a servant run somewhere and fetch a potion that they hadn't intended on using beforehand, so that meant it was pre-brewed and bottled, ready to be used on impulse. And if the memory erasing potion was on hand, then the antidote had to be cooked as well. All potions have an antidote, that was one of the things Sherlock had learned when Mycroft had forced him to take a potion brewing class. Sherlock had whined about it at first, but these last couple of months it had really come in handy. So if he could find the potions room, get past the guard and possibly find a key, then find the antidote and sneak up shore and somehow find John and talk to him without revealing his species or his hometown and then convince John to drink the potion then all would be well. Well that sounded simple, didn't it? As the days went by Sherlock got more and more confident in his seemingly impossible plan. If he took it step by step then he could probably accomplish his goal, but somehow he had to get to the first step, and that meant giving Moran the slip. The boat was still in the harbor, but Sherlock knew that any moment they might take off, and he needed to somehow get the potion and get John before that happened, because he didn't feel like going on a journey today, or any day in general. He had gotten so used to doing nothing that the very concept of swimming down the hallway seemed like an impossible task. Moran wasn't moving, somehow as Sherlock suffered and withered in boredom, Moran seemed terribly unaffected by the solitude. He managed to stay locked up with the particularly moody prince for nearly a week now, and the only conversation they ever had was the one about John. Sherlock couldn't say he wanted to talk to Moran, but it would make the whole experience a little bit less awkward. There had to be some way he could get Moran trapped, some way to make sure he couldn't notify the guards or chase after Sherlock himself. So Sherlock sat at the window, flopping his tail lazily through the water and watching the shadow of the boat. He couldn't see the dock or the people or anything, so he didn't know if they were packing up, if they had to make repairs, or if they just weren't leaving anymore. Maybe Princess Mary had died in a horrible accident, maybe she had gotten arrested, maybe...no, John couldn't have died. No. Mary could though. Nevertheless there was some sort of delay, and Sherlock was getting extremely nosey and wanted to find out what was going on. He didn't care what his reason for going to the surface might be, as long as he was up there he would be satisfied. But as he sat and stared at the boat he thought would never move, the shadow started to wander ever so slowly, the boat was setting sail. Sherlock gasped in panic, realizing that he had to go, now or never, John was leaving and if he let that ship get away he would be without his John for the rest of his life. He needed to act now.
"What is it?" Moran wondered, swimming idly over. Sherlock was panicking, did he just swim out of the room, did he attack Moran, did he ask nicely? Sherlock closed his eyes in nervousness, but made up his mind. He needed to take desperate measures, anything to get to John. So as Moran swam up Sherlock stayed quite still, watching as the boat inched farther and farther away from shore.
"Are you alright?" Moran asked, appearing at Sherlock's side. Sherlock took a deep, nervous breath, but nodded, and with that he pivoted, grabbing Moran's head in both of his hands and before the merman could react he slammed it into the windowsill. Moran slid to the floor in a daze, a thin line of blood dripping down from his forehead. Sherlock screamed a little bit, swimming back and wondering if he should reposition Moran in a more comfortable position. He had never attacked anyone before, it actually felt rather dominating. But no, focus on the task at hand, Moran wasn't going anywhere but John was, and Sherlock had to hurry.