On top doesn't work if you mean the water shader, it will black out all previous color. If you mean another plane (lake, plane, sphere, planet), yes that I can do and did, but this will slide into the planet's surface and might pop up where I don't want it. The nice thing here is that I can exactly define the upper/outer edge of where the water should end.
With a large angle between water surface and shore, it's easy, and an extra plane is better, but with the very fine 'slide' of water over the sand, this works quite well.