So i was in stormwind yesterday and fell in the water with my mount and i discovered this. When you're mounted and swimming, jumping out of the water will return you to non-swimming speed as soon as you leave the water. So if you keep jumping in and out of the water you will get through it a lot more quickly. Previously you stayed at swimming speed until you hit land, but not anymore.
So basically just hold the spacebar while you're swimming across the surface of the water and you won't be slowed down.