I made them better-

Acnologia = Shark

Midnight = Sea snake

Libby = unknown

The girls all live in the sea together while the others are normal. One day they all end up at the beach and see one of the girls before they go underwater again quickly not wanting to be seen. Midnight likes it better in dark parts of the ocean unless it's nighttime that's normally when all the girls go to the top and onto the beach just to be out of water for a while.

Its late for me so I have no clue what I wrote-