The beaches in Sosua are probably not the best choose for surfing. Too many swimmers, and no consistent rolling surf.
Playa Grande in Rio San Juan has good surfing, at certain times of the day, and there are other spots that are equally as good, or so I hear.
I'm sure the surfers (which I am decidedly not) on this board are quite able to steer you in the right direction. Whether they will or not is up to them. They may choose to keep those select spots to themselves.