depends on how well you apply it, and how good the system is.
with a good water cooling system idle temps might only drop 1-2C, the main advantage to water cooling is the fact that load temperatures drop so much... i think its because it still takes just as long for the heat to travel through the heatsink... or in this case the water block. when you start producing a lot of heat though, the water is MUCH more efficient at dissipating than the air. plus, you can add radiators.
idle might go from 31 to 30C if you apply it right... load, (though its hard to tell) might go from 48C to 35C... again, it depends on the quality if the cooling system and how well you apply it.