Another picture taken at IMTS '98. Notice the slats that it sits on. As the jet cuts, the slats get cut also, but not all the way. Part way down the slat, the jet skips to the side, and cuts no further. Slats are rotated, and flipped upside-down to extend their life.
Water is in the tank to slow the jet down.
Also the water level can be raised above the cutting surface for noise and splash reduction. I did this by moving the machine rapidly so that it would not cut all the way through. Milling is possible, but not yet a practical application. It's ok for artistic stuff, but a pain to do on complex shapes. There is a company specializing in precision abrasivejet milling.