It's a F.A.Q. obviously (hint hint, Joe: why not adding such FAQs to your site ?
)
For this you need a 12v relay and some diodes, maybe with a on/off switch (to bypass the relay when you want to test run the water loop).
You bring 12v power to the relay coil from a yellow PSU wire. Then you connect mains power to relay switch from PSU back plug (you can solder wires on it). Dont forget to put protection diodes on relay coil to avoid current problems when it's turned on or off.
Behind the relay switch you've got mains power that's turned on or off together with your mobo.
Thats it