I hope I don't sound preachy but after seeing hundreds of motors apart including mine I don't think anything can prevent carbon buildup. Your right about water injection working wonders on high compression engines, I have used it myself with excellent results. Guess what, those engines had the same carbon buildup as any other I had built. This leads me to believe that water has no effect on reducing carbon buildup.
If done carefully I don't think it really hurts anything but the potential of causing real harm if you inadvertantly kill the engine with excess water and the cold water hit a hot ring.....you get the picture. The same goes for those pour in combustion chamber cleaners, I have yet to see one that works better than plain old diesel fuel poured into a cold engine with the plugs out and cranking it with the starter.
Just my opinion.
BTW if you want a clean engine, switch to alky.....zero deposits in the combustion chamber but lots of condensation in the oil. <img src="graemlins/drunk.gif" border="0" alt="[drunk]" />
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— Han Solo