I have read about what Joe was talking about with the buckets of water a while back, apparently it does work, i haven't tried it myself so Joe is the best one to explain the buckets of water method,

With the one i have, its in the manual itself that you must wet the skin or the electrode before actually placing the electrodes on a person, i have tried it without and it doesn't work any were near as good.

here is a link were they use water on the contacts instead of gel, with a TENS machine.
http://www.sportstek.net/wireless-tens.htm

I can understand and respect you for being extra careful seeing its not your machine.