Ok seems like you guys really dont understand the concept of water.
First off its a completely different medium.
To understand the medium better you look at specific heat.
Specific heat is defined as:
specific heat, is the measure of the heat energy required to increase the temperature of a unit quantity of a substance by a unit of temperature.
Now lets look a the mediums we use.
On the air sink we have air transfering heat out via fins.
Air has a SH of about:
http://en.wikipedia.org/wiki/Specific_heat_capacity
Air (typical room conditionsA) gas 1.012-29.19-20.85
Now lets look at water.
Water at 25 °C liquid 4.1813-75.327-74.53
You see how much greater value water has over air?
This is the element we try to manipulate when using water.
Now your going to say but the water transfers to the radiator and then goes off to air.
However the average surface on a rad is usually a lot greater then an air sink.
And the waterblock with water is used to pick up the heat from the cpu.
Why?
Usually hobby rads now a days are dual pass, and have chambers for water to travel down.
Water releases the heat in a larger surface area inside a rad then a normal air heat sink.
Also water has advanced so far now, that block making has almost reached its apex.
Meaning soon, no matter what block comes out, unless its of a different metal then copper (silver or a hydrophilic carbon (which doesnt exist btw)) Your not going to see much of a benifit.
Which is why most blocks now are stagnet... there only meant for looks, while a few gives an actual perfomance boost.
Also waterblocks now a days have a very thin base, and fine cut chanels for the water to travel though.
I can go on and on about water...
But air really cant compete against water because of the specific heat values i posted above.