maybe I misunderstand you... If you seal the xbox controller won't it float?
Also I've read on various sites that some plastics do leech stuff into the water over time. I've heard a lot of people say that if you soak the plastic in hot water, and it goes brittle, slimy, or has a 'plastic' smell.. then you can' use it.
However i haven't tried this personally
number 5 plastic is the safest number 7 is the worst 1 2 & 4 are ok i dont know about 7 or 8