I have never been there, but I'll offer this up. I can't imagine wearing jeans in that enviroment, once they get wet?? yikes, heavy, chaffing,etc.
I've read that a couple of the trips, ie caving, cave tubing, hiking, etc., recommend long pants(I assume for bug protection) and a change. I have these nylon pants that are actually shorts with unzippable leggings. Get at REI for $30 or some other outdoor/camping store. Columbia is one brand. My girlfriend thought it was a "guy gadget" thing and gave them to me as a present and bought herself a pair. She loves them cuz she always gets cold at night, even in warm climates.
The shorts are great. I use them. Lightweight. Dry fast. Many pockets. Can use them for hiking or for swim trunks. Never used the leggings, cuz I thought it was kinda nerdy but this time I'm bringing them along.
Hope you have a great trip.
Of course ziplocks sounds like a good plan as well. hehe