Electronic – white powder on component leads

The easy answer is, things don't work underwater. Although technically water is not conductive (a circuit will run fine when submerged in pure water), it's all the impurities in the water that make bad things happen. And water will pick up lots of impurities. Take a container of pure water and just let it sit there, uncovered. Over time it will begin to absorb stuff from the air and its electrical conductivity will go up.

To make matters worse, water is a mild solvent so things tend to dissolve in it. And there's your real problem. You cannot predict what will be dissolved in the water. Is the pH acidic, or alkaline? What else is on your PCB that could get dissolved in the water that then effects other things? Maybe the ink on the labels of those caps could turn the pH slightly acidic, causing more corrosion on something else. Since you don't know what's in the water, you can't predict what will happen-- but only bad things can happen.

It's unlikely that you'll ever see an electrical component that is spec'd for operation underwater. The only spec that I've ever seen is in the temperature spec of some chips, that's listed as "+120 to -40 deg C, non-condensing". Underwater is considered "condensing", by the way. :)

The normal way of protecting electronics from water or high humidity is to use a conformal coating on them. That's basically a way to cover the PCB with a protective coating.

There are people who cool PCB's by submerging them in some sort of liquid. Normally mineral oil or some sort of inert chemical (the name escapes me right now). I have only seen anecdotal evidence for this, although maybe the supercomputer guys from 10-20 years ago would have done a study.

And somewhat off topic, but... While a standard motor might work underwater, it wouldn't work for long. The water, and motion of the motor, would tend to remove the lubrication. Followed by corrosion. Then the thing would seize up.

Main (and probably only) reason: copper is expensive. I've commented before that cost can outweigh other factors in design and production, and that goes also for components. Every milli-cent counts. The alloy used (no, I don't know what it is) may have a higher resistance than copper, but over the whole the difference will be negligible. I guess that for very low value resistors (< 0.01\$\Omega\$) copper may be used. Copper is also used for certain power devices because it conducts heat better. These 500\$\mu \Omega\$ examples from Isabellenhütte illustrate both: