Requirements differ depending on the size of boat, the area in which it is used, whether or not the vessel is recreational or commercial, etc. The best source of information on required safety gear, licenses, and training is your state boating office and the U.S. Coast Guard. Since half of your body heat can be lost through your head, a warm hat is an effective hypothermia preventer. Layering traps more air than single garments. Since water is an efficient heat conductor, fabrics that keep you dry or insulate when wet are the best all around for people exposed to wet and cold environments.
Insulation values vary with the type of material and the presence of water.