I would say any spot near the coast in Portugal is a good spot for Water Sport.
But please don't forget that as a wwoofer, you are supposed to be in a farm, helping and getting involved on the activities of the farm.
Some people misunderstand the concept of WWOOFing. A few months ago it was a totality wrong idea of WWOOFing in a TV channel called ProSieben in German where they classify WWOOF as a cheap holiday. WWOOF ISN'T CHEAP HOLIDAY !
If you are looking to get involved in a farm and make some water sport on your free time (sometimes isn't much) than you are in the right place. If you are looking for cheap holidays I would recommend Couchsurf or other similar system!