•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