Celestial Steed - Underwater Speed 100-150%
How to get underwater speed of 100-150%!
If you do not have flying enabled for the horse you are able to exploit the way water and flight mechanics work.
In order to do this, you must be in Northrend. While in Northrend you cannot have cold weather flying, otherwise it will not work.
This exploit is simple.
Mount up, hit the water, aim your character down, and hit space. You now "Fly" underwater at 100-150% speed. It varies, and I don't know why, however it allows you to travel VERY fast underwater