Still having problems though with some areas (see below). For example, Simpson harbour keeps the green beach no matter what I do to the map_c. The coral texture stops dead and can't be added. It must be caused by a different file methinks. Other parts of the coast have similar problems. Any insights will be most welcome.
Beaches are tricky, just like city textures that reach up the water's edge.
I think that normally one would try and accord the placement of RGB 31 (water texture) and whatever other RGB value is the beach texture. I think that in the past, I have placed the beach texture over the waters edge if you are working from a layered file for map_t.
However you can also work backwards. In the map-maker, place your beach textures with the tile option. Once you are satisfied, save all and then take the resulting newly saved map_t.tga. Extract the pixels that make up the beach textures. Extract those pixels only, none other, and then import those into your master map_t layered file at the correct layer depth so that they will remain visible. Usually that means placing them on top of layer RGB 31 for water. It seems counter intuitive since beach textures will be a lower RGB value, but whatever works is the way to go.
Or if you don't work with a layered map_t, then simply keep the newly saved map_t.tga.