A rashguard is not technically a wetsuit, but it is something which most surfers and divers wear to keep their skin safe while wearing a wetsuit. It is a very light piece of clothing which can help prevent rashes and other irritations which are often caused by a wetsuit. The neoprene and wax which are in your wetsuit are sticky and abrasive and can cause irritation when they rub against your bare skin while surfing or diving. Having this softer layer of protection will prevent this common surfer’s problem.
The rashguard is also used for surfing when you’re not wearing any sort of wetsuit at all because it can help block up to 100% of the UV rays from the sun. If you’re out surfing in the hot sun you know how quickly you can get sunburn, and even waterproof sunscreen will wear off from the constant rubbing and moving on your board.
Even though rashguard is not made with neoprene, there are some versions available that are lined with insulation to provide you with a limited amount of extra warmth. For those long days at the beach this can be enough to keep you nice and warm even against an evening breeze so you can stay out on the water for a little longer.
One final note about rashguard clothing. Since they are designed specifically to prevent skin irritation the stitching is on the outside (many first timers think this is a mistake :) ). This means that the inside of the rashguard is very smooth and won’t cause any irritation even if you are wearing a tight wetsuit over it.
If you want to learn more about which wetsuit goes with which water temperature and what are the factors that influence the feeling of cold check this article.