Water Walking is a core shaman ability learned at level 24. It gives a buff to friendly targets that allows them to walk on water. A player under the effects of Water Walking can still dive under water if they choose, and doing so will not cancel the buff.
Any damage taken will cancel the effect. Jumping into water from high enough that you would take fall damage if you were on land while Water Walking is activated causes you to take damage. This damage can kill you, as the water surface is considered to be ground while Water Walking is active. In order to avoid the damage you can aim your mouse down towards the water and hold down the right mouse button, and you will fall into the water.
Tips and tactics
Useful for escape. Where others must swim slowly, a shaman can run.
Malicious players can also use this ability to injure or kill players of their own faction by casting it on them as they leap off of a cliff into the water below.
You can use it to fish on water, allowing a safer place to stand.
You can use your flying mount while standing on water, saving you a trip to the shore, assuming you're in an area where your flying mount is usable.
You may also eat and drink without having to swim ashore.
Patch 3.0.3 (04-Nov-2008): Water Walking no longer breaks when the target mounts. It is now possible to ride on water
/Patch 3.0.2 (14-Oct-2008): Water Walking no longer breaks when the target shapeshifts. In addition, Water Walking is no longer considered a Magic buff, and thus can no longer be dispelled.