If you water cool a system, you'll have to be careful as you'll get some interesting chemical reactions going on in the conduit system. For instance, if you have copper tubing and an aluminum cooler block, copper and aluminum oxides will deposit in the cooler block and eventually clog it. If you add chlorine or salts to it to prevent biological buildup, you will be promoting the redox reactions and accelerating the chemical buildup by making your coolant somewhat strongly electrolytic. It would be wise to flush the entire system with deionized water every few weeks and disassemble it occasionally to clean off the inevitable scale. Also, anodize the aluminum parts and add a large chunk of zinc the resevoir, and that should reduce the amount that precipitates on the rest of the system, but you'll still have to clean that from to time and as the reactions occur, you'll be eating away the metals that are not being deposited on, so you'll have to replace the parts eventually. I wouldn't expect an unmaintained system to last more than about 4-8 months before clogging and a well maintained system might keep going anywhere from a few months to many years before leaking, depending on the construction of the metallic parts and electrolytic strength of the coolant.
Just watch out for corrosion.