cv643d wrote:
You fill up a bath with hot water, add soap and scrub motherboard in it, take care not to scrub components to much...
...Off course you should not drop your Amiga PSU in the bath.
I fail to see the difference?

Water corrodes the traces on the electronics. When you give it a bath, water may get under circuits, in between legs of sockets, connectors and so forth, and it takes an incredible amount of time for it to dry, and in the process eats away at the board..
I´ve seen this hundreds of times in cellular phones, granted, they arent as well ventilated, but most of them havent even been completely submerged. A few drops in the right position is all it takes.