No, a rash guard will not keep you warm in cold water. Many people confuse rash guards with wetsuits, which are meant to keep you warm in cold water. The rash guard is made of really thin material, and its main purpose is to protect you from the elements. The layer of protection it gives ensures that you do not get sunburned or stung by an organism in the water. If you plan swimming in cold water, you need to ensure that you get yourself a wetsuit. It is better designed to ensure that you stay warm while in the chilli water. The rash guard is perfect for when you are on a sunny day at the beach, it is light enough, so you do not feel like you are sweating underneath it. It also dries off fast once you get out of the water. If you want to stay warm, add a wet suit on top of your rash guard.