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.