[citation][nom]athulajp[/nom]I said long term, unless you're willing to shell out for a chiller and the power it takes to run one, liquid helium won't work long term.[/citation]
Considering the rate at which the water cooler can decapitate heat is still dependents on the size of the radiators and the the air flow provided by the fans, the lower temperatures realized is simply due to the higher heat capacity of water. The required amount of energy to raise a water cooling unit is much greater than that of a standard air cooler.
The rate of heat transfer in the water cooler still relies on the the same variables as the air cooler: difference between ambient temperature and surface temperature (delta T), surface area, and the heat transfer coefficient. A water cooler has the capacity to increase the surface area and the delta T(through increased air flow or simply, due to having greater heat capacity, having a lower temperature at the radiator). The heat transfer coefficient should be equivalent assuming that the air cooler heat sink and the radiator are constructed of the same material.
However, if you truly want increase the heat transfer rate, it would be better to submerse the radiator in a pool of water(with a current) than simply attaching a fan to transfer the heat to the atmosphere.