I think it's ok for there to be two meanings of "fish": a biologically formal (but not perfect) definition like I gave in my comment above, and a more informal meaning of "animal thing in the sea" that includes whales and even "starfish". It's very common for words to face more than one meaning. But that doesn't mean you can invalidate one by referring to the other.

I have multiple meanings of "fish", and that's ok.

But none of them include marine mammals such as seals, dolphins and whales. And none of them include penguins, even though penguins flying through the water on their little wings are impressively graceful and fast. None of these animals are fish to me under any meaning of the word.