Yes it's a funny one allright. Being strong doesn't mean you'll become a good windsurfer, so steroids are out. General fitness would be a plus, but if you're on the PWA tour I don't think that that would be such a big problem as you'd practice most days.
About the only drug that comes to mind might be a light dose of speed, but unless you take it all the time your reactions would be skewiff so it would do more harm than good.
Perhaps windsurfing is the only olympic category where no drug exists to make you better... what a wonderful sport