I have started to resin-coat certain novelty soaps. I first coat in mod podge, then I give it 2 layers of epoxy glass resin. It is looking fabulous, but someone told me that soap has too much water in it to coat in resin. What will happen to it? Will it last?
Once epoxy resin is cured, it's extremely stable and resilient against mechanical and chemical damage. It's impossible to dissolve cured resin in water, so your soap is well protected.
Adding water to resin before it's cured is a problem, though, and probably the reason why the person told you it's impossible. As a rule of thumb, if you add other substances to the epoxy resin (like paint, ink or other water solutions), you cannot add more than 10% of the epoxi's volume. If you add too much other stuff, the epoxy will never cure completely and stay soft or bendable, tacky or even runny like honey forever.