Quote:
The 220, 1000 and 4000 grit stones need to be filled with water to cut efficiently. Water also flushes away metal particles, prventing the surface of the waterstone from becoming clogged. This keeps a fresh, sharp cutting surface at all times. Immerse the stones in water for 10 minutes prior to use and keep the stone surface wet during sharpening. The 8000 grit polishing stone needs only a splash of water on the surface to cut properly; soaking is not necessary
Straight from the norton waterstones user's guide that came with my Norton 4k/8k.