What it does
Swift Swim doubles the holder's Speed in rain. That's it. No extra immunity, no damage bonus — just speed times two.
What does that mean in practice? Beartic sits at base 50 Speed, around 85 at Lv50 fully invested. In rain, that becomes 170 — enough to outspeed a huge portion of the metagame without a Choice Scarf. Basculegion runs base 75, reaching roughly 150 in rain. Rain also boosts Water-type moves by 1.5× and cuts Fire, so Water-type Swift Swim users get both speed and power at once.
Who runs it
Beartic (51%): Ice type with only base 50 Speed — unusable in the open field, threatening in rain. Icicle Crash and Superpower hit hard once it can actually move first. Ice coverage lands on Dragon, Grass, Flying, and Ground types, giving it real matchup spread.
Basculegion (14%): Water/Ghost typing is genuinely unusual — it can land Ghost-type hits that Normal-types can't block, and it's immune to Normal and Fighting moves itself. In rain it gets the Water-type damage boost on top of the speed. Less straightforward to use than Beartic, but the typing offers things Beartic can't.
How to use it
You need rain to be up. Either run a Drizzle setter on your team or set rain manually with Rain Dance. The standard line: get rain on field turn one, send in your Swift Swim user, immediately threaten faster than your opponent expected.
The fragility is weather control. If your opponent carries their own weather, they can overwrite your rain and kill your speed advantage mid-game. You either need to commit to keeping rain up or plan for what these Pokémon do without it — which is not much, speed-wise.
Tips & strategy
Trick Room is a hard counter to Swift Swim. If rain and Trick Room are both active, your 170-speed Beartic is now moving last instead of first. Watch for Trick Room leads when bringing a rain team — your rain setter should be able to threaten Trick Room setters, or you need a way to flip the Trick Room back.
Also worth knowing: Swift Swim doesn't stack with another weather speed boost. If somehow multiple speed conditions were active, only one applies. In practice this almost never matters, but don't expect any surprising interactions.