I spear/freedive in sweden and norway. I've done it in december and it gets pretty cold. The good thing about that is that visibility usually gets better than in the summer months. I use a 6.5 mm and it works fine but I think a 7 or 9 would be even better for the winter. At least for me its the hands and feet that get cold first, a good solution is bigger footpockets and thick socks, preferably open cell. For the hands three finger gloves are great. picasso and poseidon make some that seem good.
http://www.scubastore.com/producte.a...&fromhbx=frame
http://www.scubastore.com/producte.a...&fromhbx=frame
Another good trick is to drink something warm right when you get up. A long dive in cold water makes the body work hard to keep the temperature up. I think this goes on for several hours after you get up, at least I always get really tired the whole day after long cold dives. Drinking something warm gets the body temp up so you have more energy for other stuff and won't be as tired.
