There is a reflector unit very much like the air cooled "cool tube," but the bulb is in a pocket of air which is surrounded by a jacket of water encased in a tube. The unit requires a rather large reservoir, if I remember correctly (something like 20 gallons for a 400w), and the water can easily reach temperatures above 100 degrees F (I remember 120 degrees being mentioned). You'd probably need a water cooler. I have a 600w HPS in an air cooled reflector and my lamp sits 8 inches above my tops (some of my plants are as close as 6 inches from the lamp). As such, I don't think the technology is as of yet advanced enough to be cheap enough to be effective (you get wonderful results with a single centrifugal fan cooling a light, or even several). Furthermore, cool air is generally abundant for indoor growers. Growers basically just have to replace air in the reflector area with air outside the grow area. Usually that is enough to provide an effective cooling system for lighting. Even in the tightest of situations, the grower can always just exhaust their reflector rather than draw in cooler air, thus causing the lamp to be cooled by the grow room air. With the inclusion of an extra fan exhausting the grow area, this set up can even be quite effective. With the water cooled lamp in question, you only have access to water in your reservoir, which is recycled quickly, and has very little opportunity for cooling once in operation without the addition of extra equipment. Furthermore, the extra equipment, an air cooler, is generally going to take more wattage to get the job done than would air cooling.

With LEDs getting cheaper by the year, air exhaust prices remaining fairly stagnant, and HID bulbs getting better and better, I can only assume that growers will keep spending money on CFLs, increased spectrum HIDs with air cooling, and eventually, LEDs or some alternative form of grow lighting before water cooling becomes cost effective enough to be a popular form of lamp cooling.