For example, say I submerged a computer mouse entirely. Assuming that I ensured that all traces of the water were gone, would the device suffer any issues?
It is my understanding currently that provided that there is no power being supplied to a device, and assuming that non-electronic damage like rust etc. hasn't taken place, then once the device is completely dry, it can be used without issue. Is this correct?
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.
It the water is reasonably clean, that's true. I've seen a company that specialized in cleaning up after fire damage. They cleaned CRT TVs outside and inside with a hose. Then left the TV for a number of days in a heated space (50°C IIRC) to dry. They claim they have very few cases where an electronic product doesn't work afterwards.
Especially seawater should be rinsed away thoroughly. The salt it leaves behind is corrosive.
The only five instances I see electronic equipment never work again are:
The water isn't clean - one of the ones that makes a big difference, with phones, is urine. Even the diluted urine in a toilet bowl will cause problems much more quickly than fresh water. Seawater is another
You say "assuming that I ensured that all traces of the water were gone", which is a big assumption! Often there are parts of a device that are very, very difficult to remove water from. Good examples are the optical assemblies in cameras and mice, LCD touchscreen laminate assemblies, and inductors and transformers. Sometimes it is impossible to remove water even with prolonged dry heat and moving air.
The water isn't removed quickly enough and causes corrosion or other damage. A good example of this is relays - the casing often easily allows water in, but it is very difficult to get out. They also often contain steel, which will corrode and cause issues. Eventually they will dry themselves, but it's hard to get it done quickly enough.
There are components that can be outright damaged by the application of water. Paper component wrapping, some PCB laminates, water soluble coatings (such as colour light filters sometimes, or anti-reflection coatings on lenses), strings (such as on tuning mechanism on older radios - when these dry out they become brittle and too tight), anything lubricated (potentiometers, motors).
Power was applied at the time of it being covered in water.
Older equipment tended to be more sensitive to outright application of water - more components were damaged by water quickly, PCBs tended to delaminate more easily, more mechanical components. On the upside, most equipment was either mains (so turned off) or had proper on-off switches, limiting damage.
Newer equipment has the downside that a lot of things are battery powered or have soft on-off switches. Being powered up is a big issue.
