Hypothermia is the only real danger to windsurfing here. Warm head- sail all afternoon. It's like a heater. You can wear a fleece cap by itself or under the hood. They really help, too.
Good booties are nice, too.
They won't run too much. Also, for those below 50 degree days, palmless neoprene mittens work great. They allow you to grip the boom but keep the fingers warm. Don't get the cheap fisherman's neoprene gloves. You can't get a good grip on the boom and your forearms will cramp within minutes.