Dry Bags

Ultralight (UL) dry bags are used for color-coded storage in backpacks/duffels. Lightweight (LW) dry bags are used for gear storage in wet environments/paddling. Heavy Duty (HD) dry bags are perfect for demanding environments: off-road motorcycle touring whitewater etc. 

Roll-top dry bags (all brands) are not completely airtight - if you squeeze a sealed dry bag, you can squeeze air out of the closure. 

Roll-top dry bags (all brands) are not designed to be submerged - they will keep gear dry inside a boat, but if the closure is held under water deep enough/long enough water will seep through the closure. 

If enough air is trapped inside a dry bag (and the contents are not too heavy) it should float - add a buoyancy device if you expect possible unplanned immersion in water. 

Paddle-compatible dry bags have a 'D'-Ring next to the buckle - use this to secure a dry bag to a boat or to haul a bear bag/critter bag into a tree. Big River Dry Bags have webbing 'rails' on the sides which offer multiple attachment points.