I have to disagree with the moss. Many people have difficulties, as often it is either high light, or it is temperate, and will not grow year round in tropical temps (especially if collected locally). Selaginella is a beautiful plant, but it can be touchy and sensitive. I've dealt with a couple species, and they seem to thrive in one area, and die off within weeks in others.
Creeping fig (ficus sp.) grows like crazy, even in low light, and is not in the least sensitive. It may not be the same kind of ground cover as a moss or Selaginella, but it will indeed cover the substrate in nice, thick leaved vines. It will vine over into the water, or you could use your java moss around water's edge. But as you pointed out, java does need to stay moist or it will die off.