Re: getting wet
First, try not to get wet. Cover your pack in a garbage bag, cut slits for the straps if necessary. Inside the pack put your clothese in ziplock freezer bags or "space bags". The space bags are thick plastic and have a one-way valve to squeeze air out. They work great for compressing stuff, it still weighs the same though. They also sell waterproof bags for rafting, but those are expensive ($30-50 each).

Re: bathing
I have used sand to scrub my skin before. I put it in my hair also when I forgot my shampoo at camp once, and scrubbed and rinsed well. Basically you just need an abrasive to remove dead skin cells and oils. Sand worked surprisingly well.

The indians put mud caked onto their hair, let it dry and sit for a day or 2, then broke the mud shell off and touseled their hair (no water needed). The mud absorbs oils and dead skin cells stick to it. Yes, the mud might be stinky when wet, but after you remove it it should not be much of a problem. This is probably where "facial mud packs" came from.

Re: human waste
The boy scout book says dig a hole 8 inches deep, put waste and TP in there, and cover, then push a stick in the soil to mark the spot so no one else digs there. From experience, animals will dig 8 inches easily, so I suggest using a 12-14 inch deep hole.