Anyone who knows anything about windsurfing knows Maui is the place. there are several great spots around the island but I'm just going to concentrate on the best known ones. If you are looking for big waves head for north shore, if you prefer to sail in smooth waters or if you are a beginner you should stick to the southern part of the island.
South Shore is less consistent than North Shore and thus a little bit more dangerous for the inexperienced. Sugar beach is the speed sailors' paradise. You can sail parallel to the beach in flat water. It's no wonder Maui's speed contests are held here. All along the South Shore the wind is offshore so be careful not to let the wind blow you out to sea.