We just got back from the zip line excursion and yes it rained quite a bit at times. As long as you are dressed for it you will have no trouble. The only caveat is I am a bit over the 250 lb limit and one has to be very careful on some of the runs. I decided to "let it run" and does the term "a dear in the headlights" look from our guide explain to you what I must have looked like. You fly when its wet!!!!
Actually it was a great excursion. Well worth it and a lot of fun. We went on the one in Juneau where they take you on a boat to an old gold mine. Really cool.
Hope this helps.