Greg, the best advice I can give you is to give up kiteboarding imediately. It can only be doing you irreparable harm.
You should really have taken up windsurfing.

Anyway, since you will probably ignore all my good advice, here's some more.
It might not be arthritis. It could be sore from tendons which attach somewhere near there and are a bit overworked. It gives an effect like tennis elbow.
I got it some years ago and ignored it. It went away the next year.