That depends entirely on what watercooling setup you're looking at.
Megahalems is one of the best air coolers you can get, so you will not be disappointed by the performance.
Watercooling can produce lower temperatures, but you will need to spend considerably more on it. Some of the newer "all in one" solutions such as the H50 are providing a good budget starter solution for processor cooling.
You haven't provided one crucial bit of information in all of this - what is your processor?