Simple swimming is totally useless for simple travel over a large body of water. We could fly over the water and travel to other side much faster. UNLESS swimming will be used to explore underwater areas where flying mounts can't go. This is the only way I see swimming as a viable traverse mean. I also miss ships. There are a lot they can do with swimming but I'm not hyping over it. Underwater exploration is not easy to make at all if you wanna make it right. I do not expect a free roam swimming like flying. I believe its easier for them to make a "dive" area where you jump into the water watching a small cutscene of you diving into the depths of the sea reaching an underwater cavern/ruins where you can walk and breathe again. Like a fancy teleportation means. We will know more after the Fan Festivals. 4.0 will arrive Spring/Summer 2017. It's still pretty far away.