A good watercooling loop doesn't require any maintenance at all really. Even at that, the most yoiu need to do is look at the reservoir occassionally to make sure the waterlevel is ok - if it ever does get low just refill it.
A good watercooling kit will blow away any air cooling. That being said, the cost is much greater of course. I'm not talking somehting like a Thermaltake Bigwater - I'm talking about a custom built Do-It-Yourself kit. A Good kit, just to cool a CPU, is going to cost around $250. To cool CPU, GPU and Northbridge - upwards around $300 or so.