Love this thread, it put a few big smiles on my face. It's all relative. Around here, most windsurfers would regard 10 C air and water as warm enough. The best wind comes when temperatures drop a bit lower in November. We used to sail through the winter, but only on days where the temperatures were forecast to remain above freezing. Have gotten a bit soft, though, and escape to warmer climates like Texas and Oz for the two coldest months now.
The most important thing when it's cold is to dress for extended time in the water, in case something breaks down. Hypothermia sets in quickly, and makes you uncoordinated and stupid. The other thing that helps (besides warm suits and hoods) are open-palm mittens.
Here's a little video from winter sailing on Cape Cod a few years back:
We had to use a different beach to launch since our usual launch was blocked by about 100 m of slush

.