Breath is a stat that comes into play when the player is swiming and goes underwater. It is a blue meter that once had a duration of 60 seconds, but was changed in Patch 3.0.2 to 3 minutes. Players must be careful to return to the surface and replenish their breath before their breath meter runs out, especially if combat is initiated, or they will begin to take large chunks of healthdamage every second, and eventually drown. Also, if the character swims too far into open water Fatigue will also apply whether they are on the surface or underwater. If the fatigue bar runs out, the player will start taking damage and die.
While it is imperative that a player recover his breath, it can be a viable PvP tactic to stun or root players in place while underwater and allow them to drown.
Though the breath meter lasts three minutes, there are ways to counter the drowning mechanic:
Druids can shapeshift into a Sea Lion type creature with the Aquatic Form skill (Level 16), allowing them limitless water breathing. It also grants them +50% swim speed. Although they can fight in this form, the damage is very low compared to any other form except the Travel Form. (The form looks the same for both Alliance and Horde.)
Unending Breath grants underwater breathing for 10 minutes (level 16 Warlock spell, can be cast on others).
Water Breathing is a 10 minute level 22 Shaman buff, which can also be cast on others.