Map: Height Maps

[DRG]

That map you included is useless for that function as too much of the area has been set to zero elevation so it will need to be regenerated with Venhola with the elevations adjusted so your water is zero and the shorelines show up more than that otherwise you get what you show but mostly blue with a little pimple of land

.....but it only works when there is a defined shortline.... if most of the land near the water is barely above water level Venhola cannot differentiate land from water and we have no magic wand to make it do that so there WILL be cases this flood fill cannot work. There have to be contour lines above water level for it to do what you want