Water is a mechanic occasionally found on maps, whether used as a decorative element or as a legitimate hazard. Maps can feature either shallow water or deep water. Shallow water does not affect movement, but will have subtle gameplay effects, such as preventing fall damage, allowing the Shock Therapy to mini-crit, and revealing the splashing footsteps of cloaked Spies. If a player does an explosive jump out of shallow water, they will have significantly increased velocity compared to normal. It will not allow users to swim, extinguish their afterburn, or affect their weapon properties. Shallow water can be deceivingly deep, which will frustrate those who are rapidly burning to death.
Deep water allows a character to swim around and fully submerge themselves, reducing their speed, removing their ability to taunt, and allowing themselves to swim upwards and downwards. This has the obvious effect of extinguishing afterburn, but with the less obvious effect of preventing fire-spewing weapons from being fired at all, such as the Flame Thrower and Flare Gun. It also provides complete immunity from the Flame Thrower's flames, but not the Flare Gun's projectile, if fired above water. Most mechanics, like zooming in with the Sniper Rifle or spinning up the Minigun, will still work as normal. However, many projectiles will have their velocity affected by the water, and will become significantly slower. If a user holds the jump button in deep water, they will maintain their velocity for significantly longer than normal, allowing them to skate across the surface at rapid speed.
If the player spends more than 12 seconds underwater, they will begin drowning, dealing 5 damage every second (with two initial ticks of 3 damage and 4 damage) to them until their death. Once reaching the surface, they will gain 10 health every 2 seconds until their lost health is regained. There is no limit to how much health can be regenerated, so if a user is constantly healed underwater, they can take the regeneration effect on land indefinitely. The regeneration will pause when going back underwater. Players exiting deep water will have visible water droplets dripping off their character, which reveals cloaked Spies, and allows the Shock Therapy to mini-crit those players.
Buildings will still function in deep water, except for the Jump Pad, which will become flooded. However, underwater Sentry Guns cannot shoot enemies who are not themselves in water, whether shallow or deep, and Sentries on the surface cannot shoot enemies who are submerged in deep water. Unlike in Team Fortress 2, players can use teleporters underwater.