Can a device be damaged by water even if there is no power supplied?
If power is on when submersion occurs almost instantaneous damage can occur. If no power is on and there are no batteries at any stage then CLEAN PURE water will often do no damage.

A common major mistake is putting power on too soon. Days or a week of gentle drying at somewhat elevated temperature is a good idea if the equipment is valuable.

Some components or non electronic parts MAY be damaged by clean water, but often a device will survive. 



I have dropped a pager in seawater, removed battery, washed in a "clean" stream, washed more on getting home, dried slowly and had it work OK.

I dropped a portable phone in a bucket of concentrated pool chlorine, battery out, washed well, dried well, worked OK.

A friend had a small video camera with SD memory card that was in a container in the bottom of a dinghy - the allegedly waterproof container took in modest amounts of seawater while being rowed out  to a yacht. It died beyond recovery in minutes.