You can be pretty much guaranteed that as a straight coolant, water is THE best substance out there. Why? Every industry in existance uses it for cooling. Nuclear power plants are cooled with water, in-fact, and you can imagine how much heat nuclear power plants generate. The reason why water is such a good coolant is as follows: Water can basically absorb the most amount of heat energy of ANY substance before raising in temperature. This is important, because as the temperature difference between the coolant, and the thing being cooled decreases, the thermal efficiency decreases. So, even if say ammonia could absorb the heat energy faster than water to begin with, it's temperature would rise MUCH faster, and your cpu would run hotter. In a watercooling setup, you'd have to have one HOT cpu to raise your water more than maybe 5'C-10'C as it passes through the waterblock. They run supercooled ammonia through pipes underneath skating rinks to cool them basically for the reason that you can supercool ammonia, but you can't supercool water(well, not if you want it to remain a fluid).