We bought just cheap plastic ponchos that were worth only about $1 before we went and used them only for Kali River Rapids.
Needless to say we were the only people who had them on--people looked at us like we were crazy and the biggest 'nerds'(looking us up and down like we were committing the biggest fashion faux pas of the century) but after the ride when we were exiting and going to throw our "cheap Ponchos/fashion eyesores" in the garbage, everyone still waiting in line to ride were asking if they could have them!
I guess after others saw how soaked people were getting off, we weren't such a bunch of "nerds" after all!