I think manatee, sea turtle, sharks, rays and other big salt and freshwater fish would be enough to warrant aquariums as a feature.
Personally i dont really care either way for cetaceans, nothing im probably going to use often if ever tbh, but i wouldnt be bothered if they added 1 or 2.
As long as they dont make aquariums a seperate game im down
Generally I feel like you need more fish to represent them satisfyingly, than you do megafauna to represent land animals. I've tried my hand at making a Planet Aquarium roster before, and it's difficult to balance it within the scale of Planet Zoo's base 53 launch habitat species. Megaquarium has over 90 saltwater animals in its base game without touching freshwater, and even then that base game has big gaps of which not all have been filled by full-blown expansion packs.
The difference between me and others here may be that I really like fish on their own merit and prefer aquariums over land zoos. Most people here probably have the opposite preferences. But for me, I will genuinely be sad if (let's be honest - when) Planet Zoo 2 is launched with a half-baked aquarium feature. Because I know we'd be lucky to launch with more than 10 aquarium species in that situation, and that number wouldn't be substantially improved post-launch if we go off of JWE2. Maybe a couple DLCs would add an aquatic each, maybe there would be one dedicated animal pack. But that wouldn't take aquatic rep to where it would need to be for variety. Frontier and Planet Zoo players might think "you got aquariums, good enough" at that point, but the thing is that it
wouldn't be good enough, and all it would do is lock us out from getting a product that actually does celebrate the diversity of the world's waters by giving aquariums the center stage.
Hopefully I'm not too hard-line in how I come off about this. The best comparison I can think of is if JWE3 were cancelled and a handful of Jurassic dinosaurs were tacked onto PZ instead, that might disappoint fans because it'd mean that the potential that a Jurassic game would never be lived up to. Aquarium builders have a similar potential, and they are much more untapped than dinosaur park builders. The only one we've ever really had is Megaquarium. Everything else is either focused on home aquariums, or only fantastical or marine park species ala Marine Mania. We have plenty of zoo games in comparison, including plenty that mix in aquatics, yet no alternatives to Megaquarium.