I've been spending some time fixing up bits of the Gower coastline by tracing the (rather fuzzy) yahoo photos. For the most part, leaving the existing natural=coastline way where it is (which should be roughly the high tide line... ish) and filling in each beach as two areas - one inside the coastline and one outside the coastline with a water=tidal tag. I've been using surface=sand/rocky/etc. to describe the type of ground.

At the moment they are also tagged with natural=beach but I favour deprecating this tag since it is rather ambiguous and the surface tag should give enough information about how to render it.

I've drawn the tidal bits of beach down to the water line shown in the yahoo photos, which seems to be a lowish tide for most of the area. But Swansea Bay seems to be photographed at high tide, so this will be harder (I'm not wading through that knee-deep mud at low tide with a GPS! :)