Barrel shape dry bags are probably the most common dry bag style. They are cylinder shaped with an opening at one end. The seams in these dry bags are normally welded by RF welding to make them watertight. Gear is loaded in from the open end of the bag. The open end is normally closed by rolling down the extra material at the top then fastening 2 clips together.
Watch our video How to close a dry bag
to see this in action.
This method of closure is waterproof in most conditions. Is should withstand rain, waves, and if the bag is dropped in the water it should provide a sufficiently airtight seal to enable the bag to float for a period of time. Most bags with roll down tops will eventually let water in if the bag is kept immersed under water for long enough as the water gradually makes its way around the folded material. This situation is rare tough and in most situations you would need to physically force the bag underwater and hold it there to achieve this.
Some drybags have transparent viewing panels on the side to allow you to see the contents and locate your lost socks down at the bottom of the well stuffed bag.
This style of bags is popular for use as sailing dry bags.