Fanless PSUs

I see that they only really go up to about 500W. Would this be good enough to power up a generic mid-high spec rig? (e.g. i7 2700K, Z68 motherboard, 16GB RAM, Geforce 560 Ti, watercooled cpu, an SSD and a HDD, couple of optical drives). Would the PSU itself need to be watercooled?

Water cooling a psu is not an easy thing to do, you'd have to take it a part and get custom blocks made for it.

Is 500w enough for that system? depends on the psu and if you're planning to overclock.
A good 500w psu would be fine at stock speeds, pushing the overclocks up very high might give it some issues esp if you plan to overclock both cpu and gpu but high overclock are often more about the quality of the psu than it's total wattage. (although overclocking does increase the power demand)

Have you considered semi-fanless instead? that's a psu where the fan doesn't run below a certain temperature or demand level

I would more than likely be looking to overclock. I've heard of semi-fanless but don't know about the concept of them as you've just explained (probably because I was so keen to go either fanless, or standard. I'll have to take a browse for some semi-fanless PSUs

If its a good brand and you dont plan to OC the CPU or GPU too much, it should be fine.
However, if its unbranded, I wouldnt try my luck... :p

I dont think 500W would be able to run that, I reckon 700W where you want to be looking and that will give you the chance to overclock and install sli in the future