Are touch panels usable in wet conditions? Wet fingers? Wet screen?

Touch panels are not rated for continuous water exposure, mostly for aesthetic reasons (e.g., dried water leaves mineral spots on the screens). Also, it would be best to seal the screen to the bezel since water cannot be allowed to get on the edge of the panel. You can make it waterproof by using a thin (10 mil / 0.010″) clear polycarbonate layer on top. Maverick Label can make these and add a back-printed border, so you only see the active panel area. See also “Silicon Foam Gaskets” in Mounting Options.