Swimming (泳ぎ, Swimming) is an action in Super Smash Bros. Brawl, Super Smash Bros. for Wii U, and Super Smash Bros. Ultimate that gives a player mobility in water. Swimming generally has two animations: a normal floating animation and a more panicked "drowning" animation. A player that enters water will float for a while, enter the drowning animation, and eventually sink. Swim time is determined by damage; the character is irrelevant.
In Super Smash Bros. Melee and Super Smash Bros. for Nintendo 3DS, there is no swimming mechanic; instead, characters simply fall through water. In the original Super Smash Bros., there is simply no water to facilitate a swimming mechanic.
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. 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 four 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 upon surfacing. The swim time in the SSE can be increased by using stickers. In Ultimate, equipping any Spirit with the "Swimmer" ability sets swim time to infinite, allowing characters to stay in water for as long as they like.
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, making it difficult to forcibly drown an opponent.
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.)
If a character is trapped in an egg, they can sink to the bottom and be KO'd if they don't break out in time.
In Ultimate, certain characters with a weakness to water will take a small amount of damage (about 0.3% per 20 frames, stopping at 300%) while swimming. This includes Charizard, Sonic, Inkling, and Incineroar.
In some cases, a character can skip the swimming animation and go underwater. The swim timer does not take effect in this state.
Stages that support swimming
Super Smash Bros. Brawl
Hanenbow has water above its bottom blast line, but swimming in it is not possible; characters will just fall through it. Also in Corneria, the Great Fox's wings may go on water, but swimming in it still isn't possible.
Super Smash Bros. for Wii U
Pac-Land has water in one area, but swimming is not possible in the water on this stage. As one tip says: "Unlike in other stages, you can't float at all in the water here. You'll fall through it like it's not even there, so be careful not to belly flop to a KO!"
Super Smash Bros. Ultimate
Returning stages which allow swimming for the first time in the series:
Returning stages which have allowed swimming before:
Additionally, some characters now take damage just by swimming in water. These characters are: