I do both surfing and windsurfing in winter.
I can get away with a slightly less effective wetsuit surfing when the sun is out and there is no, or very little wind. When there is a bit of breeze and overcast I need the full kit.
When windsurfing, the wind chill factor is always there, but if I am very active when windsurfing and don't stop on the beach for an extended chat, I find that helps a lot.
Good booties and head gear make a huge difference for me. An insulated skiing helmet when windsurfing and a neoprene bonnet when surfing.
When windsurfing, I always use my ProLimit 'weight' vest (mostly without weight in it) as it is constructed of thick Neoprene and closed cell buoyancy foam. Both help insulate the core chest area a lot. The other big advantage is that if I do fall in, I float a lot higher, often keeping my head, neck and face out of the water, and it allows quicker water starting.
Last year I got a new GUL 4x3mm wetsuit which was on 'special' from the UK. It is fantastic and I think the thickness is a generous measurement. I wear it surfing as well but it is not as flexible in the shoulders as the Aussie surfing dedicated Wetsuits. For the warmth, I can easily accept that. I figure that Poms know a thing or two about designing wetsuits for
really cold conditions! Their summer conditions are more like our winter temperatures, and their winters are insane!
My take on it is that dedicated surfing wetsuits sold here are often made with ultimate flexibility as the priority. This is fine if that's what you are after, except that my experience is that the cell structure of the neoprene craps out a LOT quicker, the more 'stretchy' it is. I don't think the really stretchy Wetsuits last much more than one year before they are severely degraded for insulation, even if they look fine. and at the asking price for most of them they are really bad value from that aspect. When I was using them I bought the cheapest 4x3mm I could and threw them away and replaced them
every year! This GUL suit is in it's second winter and still fantastic! I would be looking at the Euro designed wetsuits, especially those orientated for windsurfing.
Wearing a 1mm short sleeved 'radiator' type vest under the wetsuit is also a great idea when it is really cold.
Last Friday it was 20 knots and 5 degrees sailing at Sandy Pt. I had full ankle covering 4mm booties and aforementioned setup on. After the couple of runs my hands were hurting bad, but soon after, my body realized it was still toasty warm and allowed the blood back into my hands. From then on my hands were fine.