The biggest obstacle in theory is underwater navigation. Currently animals only have to navigate on a "half dome", what I call a 180 degree axis, where they only have to calculate for what's in front of them and above them. For underwater navigation, or indeed flight animation, it would have to be spherical, i.e. a 360 degree axis, where the hitbox of the animal (the amount of space it needs around it) would have to constantly shift depending on what is in front of it, above it, and below it. Not to mention, for underwater especially, there would need to be triggers for when the animal breaches the surface and animations designed therearound.
There was a Q&A on Reddit some time ago before the game launched where they were asked about underwater swimming, diving, and animation, and they revealed that they had attempted it but it proved to be a more complex issue than even the climbing was, hence why it wasn't included in the base game. I believe later it was also revealed that the way water works in the Cobra Engine (which the game runs on) makes it difficult, nigh impossible, to do proper underwater navigation. The water is more of a two dimensional layer than a three dimensional space (it works for hippos, because they "walk" on the bottom of the pool and don't swim through it).
So there are some big hurdles for Frontier to jump over in order to do justice to seals, other pinnipeds, and other aquatic animals such as penguins, otters, and indeed those already in the game like the crocodilia, polar bears, and elephants.