WHO'S HERE? Sam will show up early and be around to help, including acting as a lifeguard if need be. Since he is literally Cannonball, he will help judge the contest, but I do feel like at some point he'll want to show off and do one himself with the help of his powers (will describe more under the game.) Oh, and he'll be wearing whatever kind of shorts he can find and his own undershirt (a tank top) because he's modest.
Simon might have to be forced into clothing after he starts stripping at the poolside. He'll also upturn his nose at the snacks (sorry, Vax) and probably, somehow end up naked by the end of playing chicken. (More on that under the game.)
Mal is going to be tripping balls on pixie cider while also being terrified of the water because he doesn't know how to swim. He'll show up in his regular clothes and just generally be confused about the entire thing.