As for silence watercooling can bring down the fan count in a system to just one (for active radiator cooling)
It has been proven by MikeC that an additional fan running at a similar low RPM does not increase total system noise to any meaningful degree that is noticable by the human ear at a seated position from the system.
So going from say 2X 120mm system fans running at 600rpm and a PSU fan down to 1x 120mm system fan running at 600rpm will not reduce your overall percieved noise signiture of your computer system.
My systems run with 2x 120mm nexus' at 600rpm, and then the PSU fan. You said 1 fan, i'm assuming you ment the one on your radiator, which means your system is a 2fan setup, unless your W/C'ing your PSU, which i have seen done, but its rare.
A W/C setup has everything an air setup has, except it will also have a pump. Reducing 1 or maybe 2 low rpm undervolted system fans is not going to noticably reduce your overall systems noise signiture, but the pump will definately add a new distinct noise signiture to your system. If you put a 600rpm nexus 120mm fan in your system, i guarentee you will not hear it, but you'll hear that pump when its on.
This is SPCR. This is not [H]ardOCP's forum. I'd say over 90% of the people on this forum are using systems with a total system power consumption under 200w. A W/C rig WILL NOT reduce your total system noise signiture on a computer rig pulling under 200w. So for the majority of us here using real world systems, air is king for silencing.
Bobkoure: Reguardless of if you think i did it wrong or not, my eheim pump (1280 i think was the model number), IS louder than 1. my HD's, 2. 600rpm nexus', 3. my psu. Back then i was actually using louder fans than i do now, and i could still hear that pump over those panaflo's. I did everything i could to silence that pump short of suspending it because i wanted the rig to be secure, and a dangling piece of hardware inside the case is not secure IMO.
The only time i ever hear a W/C'er saying that water is superior is when he's using some power hungry (usually O/C'd) monstrosity, and in that instance, maybe he's right. Maybe at that point, when you would have to have fans running at such a high RPM to keep up with the heat buildup that the noise from a pump would be a reduction in the total systems noise signiture. But his point is mute since the overwhelming majority of people on this forum will never have such a system.