Oil cooled PSU?
 

I'm building a custom, fanless WC setup and I want to eliminate the fan noise in the PSU as well.

I looked at some custom WC PSU setups, but the whole thing (soldering, grounding and all that fuss) is a bit out of my leaque and potentially dangerous, so I remembered the "submerged PC" idea...

Couldnt I just make a watertight plexiglass enclosure, put the PSU and a simple waterblock in it and fill the whole enclosure with clear mineral oil?

The heat from the PSU components gets into the oil, the oil itself gets cooled by the waterblock.

No soldering, no fans, no electric contact, great looks

I don't think that would work too well. Convection of the oil is unlikely to move the heat away from componentes fast enough.
However you could pump oil through the psu and cool it using a radiator. that has been done before with good results.

Note: you may need to seal some of the components against oil penetration - capacitors in particular don't react well the being submerged in oil for prolonged periods.

How do I protect the capacitors? Do they heat up and are the heatsinks in the PSU the only source of heat?

Could I submerge lika a small 300L/h pump into the PSU enclosure in the oil, so it would push the oil through the heatsinks into a tube, into a radiator and then back into the PSU?
Is such a small pump powerfull enough to push the oil?

Magdrives are very poor at pumping anything thicker than water: you'll have to try it to see.

Capacitors don't heat up significantly. You can protect them with a conformal coating or similar.