I've owned lots of Deepsea mitts over the years, they are carried in just about every dive shop around, and I have never found them to be very warm. They are cheap, so I don't mind trashing a pair in the summer time, but for winter I needed to upgrade.
Henderson has some nice stuff, but it's ridiculously overpriced for what you get. Elios makes about the warmest wet mitt I've ever used- plus, they're almost as cheap as the Deepsea stuff. I had a pair of Picasso double seal mitts that work OK, but the trigger finger on them is cut small, to use a speargun, and they are even more overpriced than Henderson. DUI used to make some nice wet mitts with a donut wrist seal that were pretty nice. They had 7mm rubber on the back of the mitts and 3mm on the plam side- with opencell rubber lining the inside. I haven't seen a pair in years, but the ones I had sure were nice.
The DUI dry gloves are the second warmest gloves I've ever worn while diving. The warmest mitts I've ever owned were some Viking lobster mitts with opencell glove liners. The main problem, besides how crazy expensive they were, was that you could only use them with a drysuit. They also were so bulky that you couldn't operate a camera or speargun with them. I had no problems using tools with them underwater- like wrenches and saws.
I've looked at some 6mm surfing mitts and have been thinking about trying a pair. They have no separate fingers and are truly a pair of mittens so they should be warm. They might not work with a speargun, but would certainly do the trick with a pole spear or Hawaiian sling. The only thing that worries me are the warnings that they are not for diving- surfing only. I'm not sure why that would be- unless the rubber is really that soft?
Sorry for the rambling post, but I've spent a long time, and a lot of money, on gloves and mitts over the years. I know that my hands get cold pretty easy, as a result from too many cold injuries from skiing, but I still possible to spend a decent amount of time in cold water with the right system.
Jon