These are all so great Tapecat, Lorna and Mark. Water is such an interesting thing, solid, liquid, transparent, opaque, reflective, muddy, etc. wow! It's kind of like fire in that it has the ability to capture you and absorb you, making you stare at it for hours. It is also inconsistent enough that you can create the look without being completely rigid in how you do it. A lazy man's dream. LOL
This is my version of fast moving water. I used an x-acto blade, an ink eraser, and a battery powered electric eraser. I scratched, erased and colored, then rescratched, reerased and more color, and more erasing. The eraser helps to soften the edges of the scratching where needed and the electric eraser removes more ink softly than a blade. It makes great spots and splashes. This is a Kayaker that I saw in Durango Colorado, from an old thread.
A close-up with color added.
Another close look.
I hope you find this useful. It really is an easy technique and a lot of fun too.