Cleaning PCBs... using water?

This is certainly possible, but a few things to keep in mind:

• Rosin flux residue often isn't water soluble. For cleaning after assembly, water is not generally effective
• Salts in the water will, especially if the pcb is left to dry in the sun and isn't mechanically dried, deposit on the PCB and ruin isolation distances and surface conductivity for sensitive circuits. To give you an example: IPA-rinsed FR-4 boards have in the order of \$10^8\$ Mohm * cm surface resistivity, a salt-contaminated board can go as low as \$10^3\$ Mohm * cm. Salt contamination is also more likely to attract local water condensation in high humidity environments.
• Crevice corrosion will occur in places where the water is trapped and cannot effectively vaporize during drying

All these reasons are why isopropyl alcohol (IPA) is most commonly used for board cleaning; it is an effective solvent for most fluxes and other common contaminants (grease, thermal compound, etc.) on PCBs, it does not affect surface resistivity and it is extremely volatile, with even very high diffusion rates and low vapor pressure at freezing temperatures.

Water has a chance of causing problems, but if used with this in mind (and certainly if care is taken to use demineralized water) it can be an effective cleaning agent if nothing else is available. Certainly, it is better than a PCB covered in dust in many cases.

As for your specific question on charged capacitors: in general, when removing electronics from its indended application either the engineer who made it has to make sure the design will eventually discharge completely and/or to safe levels, or the user needs to recognize designs not intended to be serviced and discharge any capacitors prior to cleaning. If not, water is certainly slightly conductive but not very much so. Even a droplet right in between two terminals will only very slowly discharge a capacitor with any significant charge.

What are the recommended methods to clean huge PCBs like the one in the video?

Most soldering processes involve flux that must be cleaned from the board after soldering. Some fluxes are water soluble, so yes, using filtered water is fine. Most electronic components that can be machine mounted are washable. Buzzers often have stickers covering the hole to prevent water infiltration, and other components may have warnings or special requirements for washing, but generally most components are not damaged by brief exposure to water when not in use.

Other fluxes require different solvents for cleaning.

Typically the solvent, whether water or something else, is blown off the PCB and components after washing. Air drying may leave residue on the components, and exposes the various metals to water for longer. Heating may reduce exposure, but may still leave residue.

What happens to charged capacitors when water shorts them out?

Washing is done immediately after production, and prior to testing. Typically boards are not powered up until after the board is dry and clean.

If you need to wash a board after its been powered up, for instance after rework, then you'll need to discharge any capacitors and remove, cover, or otherwise meet the washing requirements of special components.

Solder joints do not get any kind of corrosion?

Solder joints quickly form a microscopically thin layer of oxide soon after soldering due to air moisture. This layer protects the joint from further oxidation and degradation. Water, over the short term exposure needed for washing, will not cause greater oxidation.

This layer of oxide is one of the main reasons to use solder with flux.

Is the antisolder mask waterprof?

Yes, typically it is.

This cleaning process is practical and does work. Old guy I worked with used it routinely and placed boards in oven at 50degC to dry if there was no sun.

Many modern boards are aqueous cleaned so up to the task. Some are not well suited to it especially older boards with open frame potentiometers or other open components.

I should just mention a couple of things to keep in mind.

Rinsing with IPA or DI water is a good idea to get rid of any lingering dissolved salts by diluting them away.

Using oven, air or a centrifuge to speed up drying is a good thing. All water must be removed before restoring power. Connectors and any components with moving parts are at risk of trapped water.

If there are keep alive batteries then you should work fast or remove the battery. Water with salts will corrode under battery power pretty fast an keep working under ICs if water is trapped there. I do not think caps will usually be charged by the time you have a board out of a instrument but they will work like a battery if the water is conductive but for a shorter time. If DI water then the caps could remain charged and catch the unwary.

If dirt on the board is more than water soluble dust or mud then other detergents or solvents will be required. They are usually much more expensive. Clean Green or similar is a powerful surfactant that is required in small quantities. Certain board cleaning solvents use freon compounds that dry fast but seem to dissolve oily dirt but not conformal coatings. Acetone, thinners, methanol, ethanol are all options but carry the risk of dissolving some or other plastic or ink or crazing some plastics so should be used with caution and experience.