First off, I would love swimming as much as I will love flying. We have to take in consideration that Yoshi-P's team would be working on this; they will want to go the extra mile while implementing this.

They will probably add a lot of little things to go with swimming like gear getting wet (DX11!), water specific emotes, underwater treasures, new zones, new mobs, housing on the water, etc. If they implement it, they will make it a big deal. I'm sure of it.