We went out sailiing today. Beautiful, sunny, 17-knot breeze - it was delightful. Until we needed to head into the wind, which was blowing from the North and was at about 50º. The water was warmer than the air was, so the spray wasn't too bad. In December, however, the water will not be so nice to have spraying over you. Wearing ski clothes or drysuits might make it more comfortable, but sailing in winter is also more dangerous.
There are no other boats around help if you should need it. The water is cold! If you or one of your crew falls in - and your heavier clothing is going to make you clumsier - hypothermia begins in seconds, rather than minutes. Systems like your fuel pump, that worked happily in the warm summer months might decide that cold, thick diesel fuel was too much for it to put up with any more. Ditto batteries. It would be a shame to go out for a sail and not be able to re-start the engine to get back in before the sun set and it started to get even colder.
Some people do keep their boats in their slips and do go out on nice days during the winter. They tend to be quite careful, and plan thoroughly for any contingency. Your post is a good indication of the careful approach that should be taken to the subject.