A pcb in deionized water may behave differently due to the change in capacitive ratio (leaving obvious problems such as where to get pure deionized water and keep it contaminant free).
However, would covering the circuit with thermal paste have the same effect?
From my understanding, thermal paste is thermally conducting but a good electrical insulator. From basic physics, heat dissipation works better with larger surface area (and a large temperature gradient between two sources, amongst other factors such as materials used etc.
Why is this not used in complex high power circuits where passive power dissipation is required?
Would it be a problem if it was applied only across the copper tracks and on the tops of components, making an irregular heatsink that spans the dimensions of the board?
Obvious use case would be cramped Mobile devices: This would also increase the surface area, which would increase heat dissipation efficiency significantly.