Dry cags are like the top half of a drysuit. They will be made from waterproof material and normally have latex rubber seals at the collar and cuffs. They will fit around the waist tube of a spraydeck and if this is tight then it can be effective at keeping water out of the boat. If you come out of the boat and swim, water will, however, get it through the waist of the cag.
If you want to stay totally dry when you are actually in the water then you really need a full drysuit. Outside the winter months though many people find this too warm an option and a drycag is a good middle ground for months where a full drysuit isn't needed.
Keeping the cold wind and spray off you is essential when trying to stay warm while kayaking. Our spraytops are made using high quality waterproof and windproof materials to reduce the wind chill. They are designed to fit over the kayak spray deck waist tube. Our oxford nylon spray top features a neoprene collar and cuffs making it comfortable. A spraytop will not keep you as dry as a dry cag but will help take the chilling effect out of the cold air and spray.