Yup, the resistance of your skin drops in the rain or heavy perspiration. While it's a nice mitigating factor it's better to just not mess around with anything that can deliver high voltage or high current.
There's a 1999 Darwin Award for this, even. A 9v multimeter killed someone because he broke his skin with the probes and lost the benefit of that resistance. That's pretty much a worst-case scenario, but it's still a vivid reminder that "low voltage" is not "safe".