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I was told this practice originates from Taiwan and China and is linked to ancient chinese thought. According to the principle of "Wu Xing" the water is absorbed into the metal and thereafter, for a period of time, will generally make the motor run smoother and cooler. The same concepts are used in the ancient arts of chinese medicine and feng shui. Wu Xing teaches that there are five natural elements and each one has two basic interactions, metal is spawned from earth, and metal absorbs water.

I remember a trip to the ROC (Taiwan) a few years ago where a racer was describing the whole water thing to me and how a local shop would sell different types of water depending on the property you wanted to instill in your motor ('powerful', 'soft', etc.) He also seemed very interested in the placement of the receiver, battery, ESC, and everything else in his car to ensure it was aligned with the principles of feng shui.

While this has not been scientifically proven, and it is hard to scientifically prove (irreversible, testing will change performance, no two motors are the same -- a true double blind trial is not feasible), racers can be highly superstitious and techniques can be based more on folklore and regional beliefs rather than experimentation, sound science, or an understanding of physics and technology. That a racer uses these methods and they make him score better, it could well be the placebo effect depending on a racer's psyche.