Players in World of Warcraft are capable of swimming across bodies of water of sufficient depth. While swimming, the character's movement speed is reduced noticeably.
A player can move underwater by either holding the Move Down key (default X) while in the water or holding the right mouse button and pointing towards the surface of the water. As soon as the player is submerged, a gauge will appear at the top of the screen indicating how much breath the player has. Undead players, due to a Racial Bonus, are capable of staying underwater for four times as long as other races.
While underwater, almost all normal actions can be undertaken. Noticeable exceptions include eating and drinking, requiring players to find a land mass to rest on. It is also impossible to mount while swimming.
In many places, the waters are occupied by hostile monsters. This includes the Threshadons of the Redridge Mountains, the Naga of Darkshore, the Murlocs of seemingly everywhere, and numerous others. Most (but not all) other creatures will also follow you into water. Those that will not will likely Evade Bug.
It is entirely possible to fight creatures or enemy players while in the water. Contrary to common sense, spells such as a Mage's Fireball function normally in the water. It is worth noting that many mobs seem to neglect vertical distance while in the water, being able to strike from many yards below a player so long as they are near enough horizontally. Despite this, the AI will usually close the distance, allowing the player to strike back with melee.
If the player remains underwater for too long, they will begin to drown, suffering heavy damage until they resurface or die. To help with this, there are items and abilities that grant the ability to breathe underwater, such as a Warlock's Unending Breath or an Elixir of Water Breathing.
If the player swims out to deep ocean areas, indicated by a change in the color of the water on the minimap, a gauge will appear at the top of the screen that monitors the player's Fatigue level. When the Fatigue bar empties, the player will take massive damage and likely die, being too far out to sea for any hope of returning to a safe area. Note that even classes with the abilities to walk on water will suffer from fatigue, despite that they're not swimming. This is designed to prevent players from swimming from area to area, bypassing enemies along the way or reaching the limits of the world map.