Radiator vest.
http://www.radiator.net/general/shop.aspx?option=products&type=1They are pretty pricey though. I think I will look for a cheaper alternative when this one has had it.
I use a short sleeve one under my 3x2 cheapie Adrenalin wettie and add a cheapie 1mm Adrenalin sleeveless tank top vest at this time of the year. I would rather just get a new $100 wetsuit every year or so than spend $300-500 on a fancy one that will be stuffed from neoprene deterioration in a couple of years anyhow.
www.landandsea.com.au/Home_page/catalogue.htmlI also wear my thick Neoprene Buoyancy/Weight vest in cold winter storms even if I have no weight in it. It's amazing heat retention having an extra 15mm or so of insulation around the chest, and if you have to sink in the water to do a waterstart it often helps stop my head from going under.
Thick 5mm booties and insulated helmet complete the winter outfit.