Not really sure what you mean by "partial" here and it sort of depends on what you are trying to do.
Basically the waterd.bmp draws your land and also allows you to set water depth. It is the master item that gndtype.bmp is applied to.
gndtype.bmp paints this land with various textures (grass, forest, etc.). If the textures are on the land area (solid white) defined by waterdbmp then you have land textures. show to players. If they are in a water area (black) or a water area that has water depth (various steps of grey corresponding to different depths) you get a water texture being painted there and shown to players. However, if you don't have a corresponding water texture to land texture, say you just have ntt0001.bmp (grass) but no wntt0001.bmp the editor uses the land texture and basically the player would see grass under water or farm land or forest, etc.
So the interatction is that gndtype.bmp paints the land and water depth areas that waterd.bmp creates.
Shorelines can be a bit of pain but if you go with beach along the shoreline (ntt0010.bmp) and do have a wntt0010.bmp (sand or rocky sand under water texture) you don't usually run into a problem. A thin strip of beach applied to land areas and if it bleeds over into water areas a corresponding water texture is used (if you have one setup). Rivers areas can be a bit tricky though since you might not want to have a sandy shoreline for them.
However, you must remember that the water texture will only show up to players IF in your waterd.bmp file you have created water depth radiating out from the land (land is pure white rgb 255,255,255). Basically each step down from pure white is a step down in water depth. At a certain depth you will see nothing but at 254,254,254 you would see a water texture show through. If you don't have any water depth done in waterd.bmp just pure white for land and pure black for water then any ground textures you define in gndtype.bmp are only really shown to players on land. Anything that bleeds over into the water would not show because black is deep ocean (even though they actually do exist).
So many times with rivers you just don't set water depth and then don't have to worry about any textures showing through the river water. Aesthetically though sometimes we do set river depth because we want to like rocks under river water to indicate rapids or fording areas, etc. But these don't have any effect on game play are just done for aesthetic reasons.