This article explains why water droplets might interfere with touchscreen performance

You most likely have a touchmonitor equipped with Elo's IntelliTouch touchscreen technology. These work off of surface acoustic waves (SAW), anything that impedes the sound wave will cause touch issues. 

To fix this issue, make sure to prevent water from contacting the screen. In the event water does contact the screen, simply wipe it off with a soft cloth and the touch should begin working as expected.