If you are planning a windsurfing vacation on Maui check out Maui Windsurfing Guide.
On Oahu's North Shore - (very) big waves are possible, especially in the winter. Summer waves are usually small, but watch the forecast just in case.
Nice rigging area, although with limited parking. Entry is tricky - slippery ledge with lots of holes. Narrow channel through shallow reef to deep water.
Winds are best in the summer when strong convection on nearby hills results in additional air moving across beach, supplementing the trade winds.
No rental facilities anywhere close.
Kiteboarders use an area just upwind from the windsurfers. They don't have to worry about the shallow reef since they don't have long fins!