I have an idea on a boss if you plan on making any underwater areas
But when you gain access to it it will be pitch black, with only one enemy in it. A terrifying creature with glowing eyes will prevent you from seeing the surrounding area.
That is until you defeat it, the surrounding area will light back up and the monster is revealed to be nothing but a cute little Light Fish.
That's all from me though, here's some battle sprites incase you want them.