well glad you found the fix for your problem i too have issues with thick steamers impeding blood flow to the arms, as such avoid wearing it at almost all costs but have been semi hypothermc a few times!! .
As to windsurfing fitness, the recomendations here are good and i find being an avid runner and going to the weightroom regularly, that there is no substitute for just sailing sailing and sailing.
However where i live there is no sailing in winter. SO when i do take that winter trip to windsurf i have no substitute to prepare in the gym.
Core exercises are absolutley important.
The best ones IMHO done while utilizing the medicine ball, the variations are many and easy found on the net or in many fitness magazine.
Strong legs, arms, cannot be strong without the core: hips,lower back stomach being in top shape.
Suplimenting this with running or better yet swimming ( i have chased my kit in the impact zone many times!!).