Most windsurfers swear by the DaKine "Cold Water Neo Mitt
" here in Denmark. Even in Holland,most agree; I met a couple of people that said the same, while they demonized neopren gloves and especially O'Neill's Psycho gloves!
Actually any mitt is warmer than a glove because your fingers keep each other warm. Any form of insulation between the palm of your hands and the boom results in fingers and arms getting fatigued by squeezing the insulation.
Couple of downsides, if you swim a lot then you will go purple in the open-palmed mittens, while the back is typical soft neoprene and so they last only a season or two. Now DaKine's new Mitts have a new this material for your palm, thus it should help. Yet I've not tried them yet...