Suminagashi is possible on water (I think), but I do it as a marbling technique simply because marbling on a thick viscous base gives you better control over how the inks spread.
1) The base - I use Carrageenan marbling base (Carrageenan is a seaweed extract to thicken water. It comes in a powder form that you blend with water. Prepare and refrigerate this 24 hours ahead of printmaking)
2) Brush A dipped in ink. (I use Jacquard marbling ink, it is concentrated, spreads well and fixes onto fabric or paper)
3) Brush B dipped in dispersant/surfactant (to disperse the ink) (I use Jacquard synthetic gall but it is very strong, so use it 50% diluted).
4) Alum. Preparing your paper or fabric with alum solution prior to pulling your prints will ensure that the inks do not just slide off. Having said that, I used no alum in my first marbling trial and still managed to pull some prints, just with less definition.
Alternate between dipping brush A and brush B onto the surface of the water. It's a practice in steady-hands, my idea of meditative fun!
- To get darker circles/rings, you need the ink to be concentrated. And the more times you dip the inked brush tip onto the water surface, the more ink there'll be and the darker the resultant rings. Do that a few times before dipping the dispersant brush.
- Shorter time interval (faster dipping) between the dipping of inked brush vs dispersant brush will lead to tighter circles. For bigger gaps between rings, either increase concentration of dispersant on the brush or dip dispersant brush longer/more times onto the water surface.
Hope this helps! Happy printmaking ~~