So, a few points about the electricity vis-a-vis defibrillation.
One of the big concepts is that dry skin is a piss poor conductor, it's got a resistance of somewhere in the vicinity of 100,000 ohms, comparatively the wet squishy stuff inside winds up being somewhere around 11,000 ohms in the average human. Wet skin, on the other hand, has a resistance of around 1,000 ohms, and a break in the skin under the pads/paddles allows the electricity to bypass the skin entirely.
If you play around with Ohm's and Joule's laws, you'll see that for a fixed number of joules, if there is a drop in resistance, the amperage goes up. So if you have a major reduction in the resistance, you're going to wind up delivery bigger jolt to the heart, which reduces the efficacy of the shock (and can actually cause new and exciting damage).
Modern defibrillators do something called impedance compensation, which basically measures the resistance between the pads before throwing the shock, then adjusts the shock sufficiently such that even if it's not actually a 200J shock, the delivery to the heart is the same as though it were. Personally, I don't believe in relying on impedance compensation for a few reasons: First, Who knows if that 15 year old defib the mall security brought you has it. Second, It's primarily designed to compensate for something called dielectric breakdown in the skin (basically, every time you shock, you reduce the resistance of that skin), which is usually a much smaller change than wet skin. Third, I never trust my machines any more than I absolutely have to.
The other consideration is the matter of the electricity deciding to skip going through the body and instead rolling right across the skin, which would both burn the skin and, you know, not actually deliver any electricity to the heart. With modern pads, this is largely a theoretical danger only, a whole bunch of things would have to go wrong for it to happen, even if the skin was quite wet. That being said, I believe in acting as though every scenario was the worst possible one. You should try your damndest for at least a centimeter of separation from pad edge to pad edge, and at least take a quick swipe at removing some water from the skin, not only under the pads, but also between them.
tl;dr: With modern equipment, it shouldn't be any problem to defib on wet skin, but you should probably try not to, and you should try for a little distance between your pads. With old equipment there are a number of potential problems, and unsurprisingly, a whole bunch of the lore of our industry is based on old knowledge.
