I wouldn't try leaving them out on purpose but they do seem to hold up well if they get wet. I have also dropped an impact drill in a mud puddle (not the same one) and it still works. Left a cordless circular saw out in the rain overnight too and it still works also!

If you ever get an electric tool wet it is best to remove the battery (or unplug if corded) and let them dry out for a couple days before trying them. The water could cause a short and fry something if electricity goes through it when wet.