It might be worth mentioning that my own personal wishlist for the next ~35 habitat animals does include more fully-aquatic species, like bat rays, reef sharks, and sea turtles. But reducing my own list down to 20, I took off most of my aquatic animals.
If we really, really wanted to, we could condense manatees, walruses, seals, otters, etc, and just have "Slot Reserved for Marine Mammal." I mean, that gets really reductive, obviously but unless the next 5 dlc packs elect to give us more than 7 habitat animals, then I logistically don't see us getting more than 1 (maybe, maybe 2) new sea animals.
That said, the side-to-side swimming animations do already exist for crocodilians so maybe it would be reasonable to assume sharks aren't as out there as I'm thinking they are. Really I guess it's the rays and turtles with their underwater flying that would be more or less unique.
At any rate, I do feel that dolphins are highly unlikely, and orcas are basically impossible. Forgetting even the controversy of adding orcas post-Blackfish (a documentary that basically brought SeaWorld to its knees), the sheer size of a suitable habitat would basically be half the map. Now, when they tried to release Keiko into the wild, they made that giant artificial bay enclosure - and there's no long-term studies on cetaceans in that kind of quasi-suitable habitat - but the cost of making that artificial bay was exorbitant. If not for Free Willy and public outcry, none of that would have happened. Really the one and only cetacean that might ever be included would be belugas. Not saying they're not also a controversial choice, but they do seem more accepting of being in tanks than other cetaceans (probably because part of their natural habitat involves clustering around breathing holes when the sea ice traps them over winter), but even belugas seem way less likely than manatees, walrus, otters, or other ocean life.