Jack was a happy little merman and Mark was a deep sea scuba diver. Mark liked to go to the parts of the sea (not ocean) where it looked like an alien world (*cough* subnautica  *cough*) and eventually he stumbled upon Jack swimming around chasing a little one eyed green fish that didn't look like any other fish he'd seen before so he got closer and was mesmerized by Jacks tail and the way it gracefully moved in the water and the smile on his face was everything.

How will they meet? Will he be frightened or delighted? What is he like?

THIS IS AU OBVIOUSLY SO PRETEND MARK DON'T HAVE THAT FEAR OF OCEAN.