You can use full fit fins in cold water, freedivers do it all the time and I used full-foot fins for New England scuba diving for several years and still use them for freediving.
I'm repeating some comments I've previously posted elsewhere answering several questions about using full-foot freediving fins for cold water diving in case anyone is interested.
Ralph
When using heavier neoprene socks with full-foot fins you must fit the socks and fins together. Socks can be found in 1 mm to 7 mm thicknesses and Deep Thought makes some with thin kevlar soles. The kevlar works farily well but sharp rocks will tear them up. To make socks last when walking on land it's best to wear sandals or surf shoes over them. On a boat this isn't a problem.
For cold water diving you buy fins one size larger than you would use with bare feet and then fit a sock. For example, with my fins I use a 5 mm sock, 3 mm socks are too loose and thicker socks may either cause cramping or make it impossible to put the fins on. When I've borrowed Picasso fins to test, my socks wouldn't fit and I had to borrow a pair.
A couple of comments, socks are fine for boat diving and freediving from shore where I can easily wear sandals and then attach them to my float while diving. However, for scuba diving from shore in a rocky area they are just too painful to bother with. Walking with the weight of a full cold-water scuba rig, over New England rocks, with no soles on your feet, hurts. The difference between well-fitted open heel and boots compared to full-foot fins is too insignificant to endure the pain and the risk of falling.
If you want to know what size socks to use, that depends on the fins and your feet. I have narrow size 11 feet and would probably start with 5 mm socks, but that's just a wild guess. 5 mm has worked for me with both Beuchat Goldfin and Sporosub foot pockets, but not Picasso.
Even the added thickness of a thin kevlar sole may be significant. If you can't try them on at a dive shop, you might talk to the mail order dealer and ask them to send you several sizes and return the ones that don't work for a refund.
Too loose is annoying, wastes power, and increases the risk of losing a fin. Too tight causes cramps very quickly.
One other trick, I find the soles and stiching last much longer if
coated with a layer of "tool dip" or similar coating.
One more thing. You need to be careful that the fin doesn't loosen up with depth as the neoprene in the sock compresses, it might slip off. Obviously this will depend on how tight the fit is to start with and how stretchy the foot pocket on the fin is. Some freedving fins' such as the older Sporosub pockets' have almost no stretch.
Adding fin keepers which can be found at many dive shops or at Blue Water Hunter on the web will ensure the fins won't slip off.
