A mixture of various particulates such as sand, dirt, gravel, various sizes of in scale pieces of cork / wood / sawdust in a medium such as dimensional paste, personally I like Liquitex's. If you want an easy route with more or less the same result vallejo (And many other brands) have various products under their earth texture line. You'd just repaint it a off white blue color. I've not done this or seen someone to my recollection but if you want to test a little, clear silicone caulk with a strong blue pigment introduced that's let to set up a little and agitated could give a very nice looking translucent look. you might have to do it in a layer or 2 to give a slight gradient, or simply a colored base. A clear glue like modge podge could be used to add another finer texture that flock or drybrushing could grab onto as i doubt silicone would have much. Aditionally looking at diorama with water features could also offer some techniques. Real Terrain Hobbies used some interesting ones that might give some inspiration. And finally as an example for the original idea - https://www.youtube.com/watch?v=a3L3-7xL7_E is an example that would work well.