9 fans rotating at a moderate speed will move more air than 1 fan running full speed, and will produce much less motor and air turbulance noise. Also, moving an amount of air through a large opening will result in less noise than moving the same amount of air through a small one.
Water cooling is a great way to get heat out of obscure areas (such as inside an engine), but you still have to get the heat out of the water. With an engine, you have a radiator which relies on the car moving through the air to remove the heat. When you're sitting still, a fan turns on to blow air past the radiator. In a PowerMac, you'd end up with a big radiator either on top that could count on convection currents (obviating the ability to put something on top of your computer) or on the back where you'd need the fans to draw the heat out... back where we started!
In a situation where you can efficiently move air past heat sinks, water doesn't offer much. It *is* used to some degree in laptops... there are heat pipes which draw heat away from the processor and pipe it to a heat sink near a fan and vent (at least, that's the way my Pismo cools!) The new PowerMac is engineered to draw air in the front, past components and straight out the back--no turning corners (turbulance=noise), no hot-spots, just a cheese-grator look (which I happen to like, btw
Hope this helps...
[edit: added]P.S. For the record, if anyone could make water cooling practical and very cool, I'd put my money on Ives/Apple. Image a PowerMac with Wurlitzer juke-box bubble tubes!
(hey, I'm a child of the seventies, so cut me some slack!)