Rivers you can't do much with since they're just too narrow to do anything meaningful with textures and the like. The best you can hope for is to make them not blend in too much or stick out too much. If you go and do up a bunch of texturing on the ocean I think that will overpower the map and be distracting. We want to keep the eyes on the land and not the ocean. If you have access to it you might try watching something called The Color of War on the military channel, it's an American channel but you may be able to find it on the web. Particularly, look at the maps they use there...they're like this (to some extent) with a big empty light blue ocean and textured land.