Btw, this thing has been working perfect for over 4 years.
Well, I agree it's not real sanitary, however, keep in mind that even if the input was shorted, the max current flow would be 1 ma in the event of a direct short to common. Now that would probably give you a good tickle but I'm not even considering the resistance of the human body even wet. To be even safer, I would suggest that the builder (as is actually code) to use a GFCI on the power supply to the whole control panel, and this will ensure that the system is safe.
Yes the medium would definitely come into play. If your were sensing oil, for instance, I'm not sure it would work. The problem is, if you sense something that is high impedance like oil, then you would have to lower the resistance value which would then be a safety hazard.