It's against the rules to give full scripts, so I'll give you some hints...
I'll assume you'll be using GS1.
Use a clientside timeout loop to check if the player is swimming. Can use:
PHP Code:
if (strequals(#m,swim))
or:
If they are swimming, an easy way to change their spot is to simply change playerx and playery to a location on the level that is not part of the arena. If you want them warped to a different level, you will have to use triggeraction, since setlevel2 is a serverside command.