So, basically right now there are only three creatures that you can fight, sharks, kraken and meg (not counting sirens)
How about adding some more, like
Whales (also maybe a boss variant of a white whale or something dunno)
Jelly fish
Manta rays?
Also the thing that I think would fit the most
Crabs
Just normal crabs that can be underwater / walk on islands and maybe a boss variant of a giant crab?
Dunno, there's a lot of things I could say but that's the most I want to say right now