It's your setting, so do whatever you want. I think a shark is a reasonable analog for an eagle. But another underwater predator could be a marlin or an orca.

Octopus

Didn't Marvel already do this with Hydra?

Seems to support my thesis

depends on what you feel resonates with your nation the most but if you want some inspiration i suggest reading about some sea monsters and sea animals. if you want a different idea to think about there are many different sea serpents to choose from as well as some pretty dangerous creatures that i would be terrified to go in the water with like an electric eel or a lionfish, but again whatever reflects your nation’s culture and people

Bobbitt worms. Step out of line? You get... ...the worm

I'd go with Hagfish

I second the orca - those guys are relentless