In some of the high-altitude lakes and ponds in Mephalan Vales, the fake water is placed atop terrain using a grass texture. I don't think the grass would survive very long in these conditions, so it would more logical to use a mud or underwater texture. This would also prevent groundcover mods from placing grass under the water. One example of this is in the lake stretching between (28,-14) and (29,-14), but there might be more occurences.