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.