I've not been able to justify watercooling. The risk of failure is what keeps me away.
If there's any failure of equipment or user error, water could destroy the entire system.
If the fan on my slk900 fails, the system will overheat and shutdown. No big deal.
Plus watercooling is relatively very costly if you want quality equipment (which is a good idea for the above reason).
And its really not going to be quieter, considering that fan noise can be eliminated by a rheobus & quality fans. HD's and CD drives will make the same noise an air-cooled system and a water-cooled system.