I too have gone the water cooling route, using some Mayhems coolant and a variety of pumps, radiators, tubing and waterblocks. It works really well with my graphics cards rarely breaking the 40C barrier no matter what the load.
Big fans rotate slower (quieter) and shift lots of air. If you have a massive (think car radiator size) radiator with a single massive fan it is very quiet indeed, and very efficient.
Full immersion is not for the faint hearted. You can go for deionised water, but it quickly gets contaminated and conducts. Nobody wants that. Mineral oil sounds like a messy solution to me, especially if it was being used as a heat sink rather than via a radiator. Oil is much, much harder to pump than water.
BUT
When you take the fans out of the case, including the gfx and cpu fans, there is now little to no air movement over components that would normally be passively cooled by the air currents those fans would generate, so you still need some air being shifted around. This is the issue with fanless cases, odd little bits of your motherboard overheat because they can't shed the load as their design requires air movement over them.
Trying to cool below ambient is a bad idea unless you really know what you are doing - you will get condensation.
The quietest PC is the one in the room next door, with a couple of holes drilled in the wall for cables.
