They already worked out how to make pets swim
I have just the basic notions(Very... VERY... basic notions) of how things work, and methinks that, we already have us, the MC being able to swim, the mechanics are there, and I think(Might be wrong, but I am 50/50 certain I am not) that they could just adapt that to the companions, I'd say NPCs but then some people might not like mobs following you through water(Though I would, find it silly that you can shoot them out from the other side whlie they run, and in Skyrim and Oblivion mobs do swim)
So I really hope we get that, I love companions chatting you up(Even though they have limited lines, I would defo be onboard for a crown pack of more lines for conversations on all of them tbh haha) and find it annoying that if they are mid comment and you accidentally(Or not) drop to the water, poof, no more comment and they just vanish
Please, pretty please, adapt the toon swimming mechs to companions!