Will you be doing a lot of line handling, or just steering
for the most part?
I use kevlar reinforced snowboarding mittens down to the 20 degree point (never moved the boat in less than that).
I was an avid (and semi-pro) snowboarder waaaay back in my teenage years and have used the same gear
from that sport for sailing in the cold weather
. It's all pretty waterproof and breathable.
This particular example may not be the best brand or quality, but it popped up first on my Google
search for an example:
Level Matrix Snowboard Mitten
They have kevlar built into the palms and thumbs to keep the mitten from falling apart. I have pairs of these from the 90's that are still going strong after years of snowboarding and sailing in them. Pay a lot of $$ for them and you won't have to replace them. I think mine were $80.