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 
Watershed makes good bags. You likely are well aware of this, but I'll mention it because I so often see it happen is people struggle trying to open the rubber zip sealed bags by pulling them straight apart. WRONG!
Grab the loops on other side of the seal and pull them sideways towards each other and it opens easily. I've suggested that they put instructions on the bag, and they tell me they have considered doing that, but so far they haven't yet done so.
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