Unless I'm forgetting someone, the characters in Smash with swimsuit alts are just

and

(on a technicality due to the goggles alt).

can kinda-sorta count if you include her shorts and crop top/sports bra alt.
As for characters who have canonical swimsuits but don't have them in Smash, we have





and the female versions of

and

. And of course the man himself
even with his Hank Hill curse.
Other fringe cases include shirtless versions of


and

, as well as the broken armor versions of

and the male versions of

and

.
Even then I'm probably forgetting some.