Hi.... infact there are small small beaches between the groins. 
Drawing beaches on top of the water has no use ofcourse. No rendering.
But changing the coastline is tricky for a starting mapper. Broken mp's, crossing and self intersecting polygones.
I created some beach examples but had no reply fom this mapper.
He started again editing the coastline and created new beached further inland.
Certainly Pokemonedits.