Not just in WoW, but in most games it's just awkward. You have a game built to be played at large on a 2D plain (running around and jumping in a 3D world), the player then spends 90% of their time on said terrain, and then you toss em underwater which is suddenly a full 3D plain (by which I mean full movement control in all directions, aka swimming). It's jarring to your play experience, and the game itself usually isn't all that well made to accommodate such movement. Even if it works perfectly fine so far as smoothness and tightness of controls go, it's just not designed to be played as a game in a 3D plain at it's core (such as a plane game where you fly around trying to shoot each other down for example).