Answer:
Well I think there are many things you could right about! Coming up with a fictional ocean creatures are easy! Like when you think about it, you could write about a terrifying octopus hunting people in the sea shores or you could right about a mythical mermaid and and cool tale about that! (I'm kinda just making up options up for you! so really you could search up a sea creature in your desire and then write about it!)
Lemme know if this helped a little!
Noriya~