I doubt it. Swimming isn't something you have access to right away in New Leaf. You needed to get a wet suit, which didn't happen until you got to the island. I bet it'll be unlockable in this game too.
Pretty sure it's still a feature in the game, they probably not mentioned it as it wasn't (in their eyes) an important thing to show and wanted to focus more on the newer features the game has to offer. After all, why should they removing it if it fits the best to the game's theme?
I'm wondering how they will restrict us from swimming up the rivers or if we will be able to swim in them. I think I may have seen some rocks that could stop up but I'm not sure
I wouldn't make any assumptions really. I personally am a bit neutral on the swimming. It was a fun little activity but also quite restrictive. We'll see in the end. I mean, I kinda hope the direct left at least a few surprises in the game