"As ships sail over a layer of warm water sitting over saltier, or colder, layers, waves form in the boundary between the two layers. As these waves grow, they form a gulf beneath the ship, sucking away its speed. This effect can stall boats at sea, reducing their speed by up to 80%."

It might affect the fin to some degree, although the fin is generally in a different plane to the waves.

It would seem to only really affect displacement vessels, so the stratified layers would have to be extremely shallow to have any effect on a planing windsurfer. Surface chop would have a far greater impact.

To my way of thinking it would only affect displacement hulls/bodies?
Not enough board in the water if planing.