Swimming (泳ぎ oyogi) is an action in Super Smash Bros. Brawl that lets a player swim in water, instead of falling right through it like in its predecessor, Super Smash Bros. Melee. The time for which players can continue to swim is determined by how much damage has been received: the more damage accumulated, the less time the character can stay afloat before drowning. Drowning causes the character to sink through the water while immobile and be KO'd. It cannot be stopped once started. The game keeps track of the number of KOs by drowning. All characters have the same base swim time and can drown, regardless of swimming ability or underwater survivability in their own games. However, characters like Squirtle swim much faster than characters such as Sonic.
Characters can swim for a maximum of 4 seconds, after which they can remain afloat floundering for an additional 2.5 seconds, when they will enter a "panicking" animation. The 4-second clock is shorter for higher-damaged characters, at a rate of 1 second for each 24% damage. As a result, characters at 96% damage or higher will enter the flailing state immediately on surfacing. The swim time can be increased by using stickers.
Swim times are only reset by landing on dry land; simply jumping out of the water momentarily will not give the player any extra time (although it will stall the amount of time). However, it does reset it to the "panic" stage. The player can continue jumping out of and back into the water infinitely and never drown, provided they don't wait over the normal "panic" limit. Even characters with 999% damage have 2.5 seconds before drowning to jump, meaning it is practically impossible to force an opponent to drown.
Apart from moving, characters can only jump when swimming, which mostly acts as their ground jump.
Interestingly, swimming times are unaffected by transformations; a metal character can swim just as long as a normal character. That said, a metal character will sink further before surfacing. (This trick is used once in the Subspace Emissary in a secret room.)
A character hit downwards towards water by a meteor smash may be KO'd outright if their downwards speed is too fast for them to swim to the surface.
If Wario is riding his bike, he will fall right through the water. Letting go of the bike will cause him to swim and his bike to sink.
Characters using Final Smashes (including some transformations such as Giga Bowser) will fall through water and be KO'd. The exception is Mr. Game & Watch's Final Smash, who instead of falling through, he will be able to float permanently on top of the water and be unaffected by currents during the entirety of Octopus, including those on Jungle Japes.
If a character is using a longer Tether Recovery, he or she will pass through the water as if it's not there until the character stops grabbing the edge.
In the Subspace Emissary, if Player 2 sinks directly through the water after he or she panics, this will trigger an automatic Space Jump.
 Stages in Brawl that support Swimming