Starting with Super Smash Brothers: Brawl, if characters fell into bodies of water on certain stages, they had a few seconds to swim out before drowning. Being in the water doesn’t hurt, although if characters complete their drowning animation, they plummet to the bottom and die.
In Super Smash Brothers Ultimate, that’s all still the case, except now, there are four characters who accrue damage just from being in the water, even if they get out before drowning: Charizard, Incineroar, Inkling, and Sonic.
The amount of time that every Smash character can spend in water before they start to drown is dependent upon how much damage they’ve accrued before falling in. If you have over 96% accrued damage, you’ll start drowning as soon as you hit the waves.
If you hop in the water with no damage, you’ve got four seconds before the drowning animation begins, after which point you have roughly two seconds to make it back to shore. That’s all also the case in Brawl and Smash Wii U.
However, in Ultimate, four fighters are weak to water and will accrue about 1% of damage per second as long as they are swimming. It’s not much, but it’s not nothing, either.
Charizard and Incineroar’s weakness to water makes sense, given that both characters are fire-type Pokémon against whom water is “super effective.” Similarly, the Inklings can’t swim in water in Splatoon, although that is a bit odd given that squids are their evolutionary ancestor.
Sonic also can’t swim in his own games, although that came about because of a misconception: Yuji Naka, one of Sonic’s creators, told VideoGamer that he had mistakenly thought that hedgehogs couldn’t swim.
Turns out they can, but Sonic still can’t.
Although Sonic and the Inklings do accrue damage in water, which makes a certain kind of sense, it doesn’t make sense that they know how to swim in the first place. It also doesn’t make sense that Squirtle can drown. I’m starting to think that Super Smash Brothers might not be a very realistic game.