winglets on fins mostly don't work because the pitch angle of the board is not stable enough.
a winglet which is out of proper alignment will have a lot of drag.
on speed or most windsurfer fins winglets don't make sense anyway, because at higher speeds the the lift coefficient is very low and so the induced drag is quite low as a result.
winglets are not the optimum solution to reduce induced drag if you don't have any span restrictions. on planes they are often used to squeeze more lift from a wing without needing a new wing design and also for directional stability.
www.mh-aerotools.de/airfoils/winglets.htm
cheers
Boogie
