Interesting question. Regarding water-solubility, consider Daguerreotypes. Water doesn't even figure in to these; so maybe there's some way??
not true mon frere. You still have a water-based fixing bath, a post-fix wash in distilled water, and the gilding stage is also 5% gold chloride in an aqueous suspension. Plenty of opportunities to dissolve the light-sensitive silver halide layer were it water-soluble.