Y'all remember Thundercats? Well, Tigersharks was pretty similar, and from the same creators.

Look it up, but what I'm thinking is more inspired by the original rather than trying to recreate it outright.

The premise is that the main characters get whisked away to a pocket dimension on earth; the true location of Atlantis! There, they discover that they somehow have ancestral ties to this other world, and somehow, something opened the gates between worlds, drawing them back to defend it! Among other powers, each character would be able to transform into humanoids/humans with traits like sealife.

Special powers would not be accessible without transformation, and the transformation can't be sustained indefinitely.

Here's the intro for funsies.

https://youtu.be/J_GWnCOYk2Q?si=Mw72jrgRvjdTQcdK