I would imagine that the proper insulation layers would be dependent on the temperature and how active you are. Those large insulating one piece undersuits are great for diving and some active surface use, but are just too bulky for me. Most of the hydrofoilers I ride with all year use layers from NRS Apparel > Base Layers > Midweight Layers at NRSweb.com
They are compact, stretchy, snug fitting, warm and won't hold much water if you do burp in water or get a leak. A lot of layers and a life vest can make it very difficult to climb in and out of the boat and as Jyaski says anything that soaks up moisture is a poor choice. Dress for your activity level and don't get so bulky that you can't swim decently.