I've water washed boards for years.. It depends on the board and how it was assembled. Low clean no clean and water clean fluxes are both water soluble and hygroscopic to some extent, Some worse than others. All components are "Generally" watertight the notable differences being sensors. A crystal or resonator must be watertight in order to avoid the influence of the flux used in soldering the board and the board cleanup later or be susceptible to changes in frequency because of foreign material buildup.
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