I agree that it has the potential to be a tricky subject. As it goes, there are arguments against keeping many animals in captivity - elephants are the big one recently in the zoo community - that have to be carefully tackled in games like these. When Marine Mania was released as an expansion to zoo tycoon 2 there wasn't nearly the same level of controversy surrounding killer whales in captivity as there is now, and it would be understandable if that swayed the devs decision to include them in the game at all.
From what I've read from those who played the game pre-alpha, the needs of the animals are complex (a la JWE, I presume, but maybe even more detailed than that?). I think a good way of tackling these controversially-kept animals ingame would be to make them high-risk, high-reward. That being, high requirements to keep them happy, but much to gain in terms of guest satisfaction, zoo fame, breeding success etc. if they are. I suspect the game will penalise you for poor welfare regardless, just as PlanCo penalises unhappy guests and JWE penalises poorly cared for dinosaurs. Personally I would like this to be deeper in Planet Zoo, for example animals being confiscated if they aren't cared for properly, animal rights groups protesting certain practices etc. though we will see how it goes on that front.
Tl;dr If handled properly killer whales would be a fun and even educational addition to the game.
In terms of killer whales being in the game, though, I doubt we will see many aquatic creatures in the base game aside from perhaps penguins and polar bears. I am 99% certain we will get a marine expansion down the road, though, a la zoo tycoon, wildlife park etc. It makes sense for the devs to play it that way.