Sailing exposes you to the elements where conditions can often change quickly. Having waterproof gear on hand is a smart move in case you encounter adverse weather.
Waterproof sailing gear should be both waterproof and windproof. It shouldn’t be too baggy, but there should be enough room to be able to stand, sit, crouch, and move about comfortably. You’ll also want your waterproof gear to be lightweight so that you can add it on top of your warmer layers. Some waterproof material can be uncomfortable against your skin, so you may want to wear a rash guard or technical shirt underneath.