12 species have Swift Swim as a hidden ability, 32 as a normal one

Effect, in PokéAPI's words

“Doubles Speed during rain.”

As a hidden ability

Species	Slot	Note
Psyduck	Hidden	Since Generation V
Golduck	Hidden	Since Generation V
Poliwag	Hidden	Since Generation V
Poliwhirl	Hidden	Since Generation V
Poliwrath	Hidden	Since Generation V
Anorith	Hidden	Since Generation V
Armaldo	Hidden	Since Generation V
Tirtouga	Hidden
Carracosta	Hidden
Beartic	Hidden
Chewtle	Hidden
Drednaw	Hidden

As a normal ability

Species	Slot	Note
Horsea	Normal
Goldeen	Normal
Seaking	Normal
Magikarp	Normal
Omanyte	Normal
Omastar	Normal
Kabuto	Normal
Kabutops	Normal
Qwilfish	Normal
Mantine	Normal
Kingdra	Normal
Lotad	Normal
Lombre	Normal
Ludicolo	Normal
Surskit	Normal
Feebas	Normal
Huntail	Normal
Gorebyss	Normal
Relicanth	Normal
Luvdisc	Normal
Buizel	Normal
Floatzel	Normal
Finneon	Normal
Lumineon	Normal
Mantyke	Normal
Tympole	Normal
Palpitoad	Normal
Seismitoad	Normal
Arrokuda	Normal
Barraskewda	Normal
Basculegion	Normal	Since Generation IX
Overqwil	Normal

Elsewhere in PokéAPI

4 more holders without a page here

Alternate forms and other species that PokéAPI lists with Swift Swim.

Swampert Mega, Drednaw Gmax (hidden), Qwilfish Hisui, Basculegion Female