They migrate just like Orcas. They probably need a larger area of water.
There is a difference between "can't be kept adequate in captivity" and "can't survive" though. There are a lot of animals that can survive in captivity but suffer. And then there are animals like the white shark that just were never successfully kept in captivity.
I'm all for advocating to not keep whales and dolphins in captivity, but we should keep it to facts.
That said, personally, even if we let whales and dolphins out of the discussion, in my opinion an aquarium DLC would stray farer from the game than most other DLC ideas. Aquariums are often seperated and not neccessarily essential for a zoo. We got a bunch of zoos without fish.
And as I said elsewhere, the free swimming of the animals we have already has it's issues, same as free climbing. I'd much rather have looped birds in this case, but unless a 20+ huge DLC that too seems kind of pointless by now. Like, there wouldn't be enough variety anyway otherwise and it just would fire up a discussion about "why that bird and not this" because there wouldn't even be a chance of getting similar / other birds next. I know what I'm talking about, I can safely say my time with the Giant Otter vs. Asian Small Clawed and Seal vs. California Sea Lion etc. was frustrating.
I don't know, if we really have only 1 DLC left, I think Frontier should not open new "concepts" and that includes basically anything that is much different from the current roaster. Fish, flying birds, fully aquatic mammals, petting zoo animals (though I admit an animal pack would maybe do them justice enough, I personally still wouldn't put my vote on it).