People generally believe water cooling is more efficient then forced air cooling, which involves all three modes of heat transfer.

Convection is not too efficient(dead air pocket consideration), conduction is very efficient, and radiation is known as infared wave length transmission(reflection and absorbtion).

Water coolers have two parts, the heat sink and the radiator, which uses forced air for cooling. The transfer medium is water or antifreeze(more efficient conduction).

The present breed of water cooler has too large a mass for heat sink contacting the cpu. So heat retention is a problem. Water pipes are now 1" diameter, which presents a large mass and difficult to heat up and have more efficient cooling at the rediator.

The rediator must have very small tubing(3/16" diameter max) and large air fins(by pressure contact) to cool the small tubings(copper). Then a thin pancake fan to cool the fins(aluminum).

The next generation water coolers will hopefully be more efficient and small in size with bigger radiator sections that can be added at will, section by section, each with its own fan cooling. Hopefully, the temperature at the radiator will be higher so cooling is more effective.