We always have some small bills in our beach bag and have never had a problem. Wet bills are never a problem at the bar if your cash does get wet. Cash and your phone/camera/room key can be a problem on Lingerie/casino night. Not many pockets in most boxers:azzangel:, and the gals definitely don't have pockets.
I think it's just because coins are heavy and not worth that much in comparison to bills of a similar mass.
Better then the straight exchange they give you in a lot of Mexican places There's a 5% exchange charge ( Canadian dollar has gone up last few days)
For 7 days I bring 2000 pesos in small denomination for tipping and 2 to 6 hundred is US funds, depending on how much bar hopping and excursions you want to do. You can pay for most excursion with your Credit card.