I would never trust any drybag 100% if it's going to actually be submerged, even if it's an expensive one.
I've bought some of these, though, because they'll do well inside my various other barrels and drybags. A drybag in a drybag is properly waterproof. These look like they're quite thin, so I'll be hedging my bets by hiding them inside my nice Lomo ones.