I have been windsurfing since 1977. Like sea kayaking you can do it in a variety of conditions but I find that the two sports are highly complimentary and tend to windsurf when the wind is stronger.
Quite a lot of the time you can do both and a speed comparison is interesting. There was a 12 to 17 knot wind blowing straight down the bay to the sea. Paddling the mile across the bay at right angles to the wind, I averaged 2.9 knots.
Repeating the exercise on a light wind windsurfer with a 7.0m sail I averaged 25.1 knots. It is a great feeling flying along faster than the wind, with only the skeg in the water. I am surprised more sea kayakers do not windsurf.