If you need a good underwater ditch bag, take a look at watershed bags. I use them extensively on all my gear, in my pack,etc.. These bags are tough and are rated for 200 feet underwater with a lifetime warranty. I would recommend getting a purge valve with it if your interested. I carried these while traveling and could keep control of all my gear when swimming. Your gear won't get a drop wet with these bags.

http://drybags.com/home.html