I've seen those in marine stores. They are, IMO, about as waterproof as a paper bag. The plastic is too thin to keep shape when under compression, which is required to seal against the O-ring seals.
A much better choice would be a Pelican box or similar.
I've got one of those. Bought it at the boatshow from solent plastics to keep my camera dry when out in the rib.
Its cheap for a reason, it won't withstand heavy pressure and i doubt it would cope with a proper dunking but it seems pretty good value to me. I can just flip the catch and have my camera out in seconds.
A pelican or storm case would give much better protection but you have to pay for it!
I've got one, again from solent plastics in soton (nice people), it's better than a standard toolbox but it's no peli case! However my tools are in it on the rib!
