Be carefull with deionized water as it will suck the ions out of any metals it comes in contact with. Deionized water is pure H2O molecules, which sounds good, but has no mineral content to neutralize the water. You need SOME mineral content or the DI water will leach the minerals right out of the cases. End result is holes in very expensive parts!
Distilled water is a much safer liquid for cooling (with aprooved anti-freeze, of course).