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Single closure roll top dry bags typically let a bit of water in if completely immersed. Outdoor Research has some double closure windowed dry bags that are actually pretty dry. I have been told that the ziplock type closure on Watershed dry bags is pretty watertight as well.
Five gallon buckets with gamma lids are also typically not fully waterproof unless you seal around the junction of the ring and the bucket with something like silicon sealant. The O-ring on the screw down hatch is reasonably water proof is screwed down tightly.
Best in terms of water tightness are either dry boxes, such as those made by Pelican, or the 30 or 60 liter plastic blue barrels.
I'm told by my friends who have tripped on the Colorado River that the Bill's Bags are also pretty good.
I have used 30L, 60L, and 80L blue barrels, 5 gallon buckets with gamma seal lids, and dry boxes. I like to use roll top dry bags inside of a portage pack.
Have used both, generally speaking I have had good luck with both; which is to say that I have personally never had real issues with either.
Bottom line: If I have any concerns at all about keeping any particular items dry(especially electronics/first aid gear), whether I am using bags or buckets; I back up the bag or bucket with secondary protection. I might use a waterproof map case, garbage bag, or a Pelican box as back up. Depends on the item in question.
Under the wrong circumatances; anything will leak.
Ask the Captain of the Titanic.
P.S. Listen to pblanc's (good) advice about using sealant on 5 gallon buckets.