I have a Canon A310 (2-megapixel digital elph model) that I have used for both scuba diving (with Canon's underwater housing) and caving (no housing). Caves with any amount of water can easily reach 100% humidity, and are also notoriously dusty. I'd never take a digital SLR underground, but my little point-n-shoot has served me faithfully for years.
As far as conditions on the water, I'd highly recommend an underwater housing as opposed to a plastic bag. Yes, the housings are a LOT more expensive, but they're also designed to be totally water-proof and have clear optics where your lens goes. Plastic bags can leak (trapping water *inside* the bag) and might fog or blur your photographs if you're trying to take pictures through the bag (I haven't tried this, though). Plus, with an underwater housing, you'll be able to take pictures of any fish, corals, and even family members underwater--it will open up a whole new realm of picture possibilities. Underwater photography is a lot of fun.
I'll second the advice to be careful of large temperature changes where humidity is present because water condensation is bad news. When I'm going diving, I'll wait until I'm on the boat or at the dive site to load my camera into its housing. I want the camera and housing at the same temperature when I load it to avoid fogging the lens. There can be a bit of trial and error invovled, and I have had fog spoil some of my shots in cold water conditions, but it hasn't ruined my camera. In the warm waters of Mexico you should have no problems at all.
For caving, I use a Pelican Micro case, which is a rugged waterproof case that protects my camera from moisture and shock while it's not in use. Another brand of similar cases is the "Otter Box." You should be able to find these online, at scuba shops, and maybe at some electronics dealers. Sizes vary, so bring your camera to see which case will work best for you. I cut out a couple small pieces of mouse pad so that my camera fits snugly in my case. I don't use any additional precautions when shooting pictures underground--just pull it out, snap away, then put it back in the case. I use the same case for general travel, too, and never worry about dropping or damaging my camera.
I hope you find some of these ideas helpful, and enjoy your trip!