I can't really vouch for actually building a swim platform out of Trex, But I would stay away from the Trex brand itself. They have over 4000 warranty issues to date. I have replaced 6 decks this summer and all were Trex.
As far as building one and would it work? I don't see why not. You would need to find a brand that doesn't have a shaped bottom or holes running through it and use good quality stainless hardware. I would also stick to a light color if possible to keep the heat down. Some of the darker colors can get pretty warm to walk on. Evergrain is pretty popular up here in North Idaho and hold up well to out cold winters and hot summers.
http://www.tamko.com/Decks-Docks-Rai...mpositeDecking