yep, watter is quite hard to manage and understand, i think it must be the hardest to get. I think the better way is to let the hand get the flow, the mouvement, the chaos, to not try to controle it. Once you have the primary shapes, let the inconscious built the reste. It's like any other landscape, but with more visible mood and lightning games. ^^
that's something i really love to draw or paint.