Thanks, Sandy. I did not know exactly how Oxaca did their water. I imagine that there are some sort of water lines but they are so bad that they've resorted to cisterns. I understand some hotels in San Miguel use cisterns, too, because of the plumbing issues and it's a very old town, too. Of course I mean no denegration towards Mexico about this. I love Mexico, but reality is reality. Interestingly, one can go to Hawaii and live in areas that have no water plumbing, too. I stayed at one vacation home that used a 'catchment' system and I got sock from that water, too. I went to Indianapolis and got sick from city water.
Water\food concerns aside, my comments about photography would be to not forget the opportunities of night photography while there!! Often, night lights in Mexico are plain bulbs hanging from wires. These tend to throw very sharp and dark shadows on walls and streets. This can be very dramatic. Check out my gallery for some night shots. I believe I shot mostly with 30-second exposures using ASA 100 film. I used a hand-held Gossen light meter as it was more sensitive to low light than the Pentax 1 Deg. digital meter was. I would set my camera up and then walk into the shadow and meter while I was in it. Also, if you are photographing a dark scene and it's hard to see what you're focusing on, you can carry a candle, set it next to the subject and focus on the flame and then remove the candle before the shot. A flashlight works well, too. I took with me some Rodinal, fixer and minimum equipment necessary to process film. I would develop the film the next day and go re-shoot anything that was under\over-exposed. This is extrememly important to getting good night shots - finding out the negatives are bad when you get back is too late!
I never really shot people a lot while there. I got mixed reactions from people when I tried. You may get feedback from others here about that. Other than thatn, I pretty much roamed freely around and only got run off a few times. Once by a guy with a 6-foot long machete with the last 3 feet painted red - at 3am in a remote part of town. I tried to talk him out of it, but I didn't speak Spanish, he didn't speak English so the international language of the machete prevailed<g>. Cemetaries can be really interesting. In San Miguel, they had human bones stacked up at the rear of the cemetary. As I was told, people would rent a grave and if they didn't\couldn'gt keep up with the payments they dug up the body and tossed the bones and rented the spot to someone else. They may have burned the bones or gave them to the family if they were around. I don't know if it's true, but there were a lot of human bones there (a doctor with me verified they were human).
Churches are great, too. There are a lot of icons indoors and 'suffering Christs' all over the place. Look for 'God rays' coming thru high windows. Some churches have interesting udnerground areas, too.
If they have bullfights in that town, it can be a good place to photograph.
There should be lots of cactus around. I have a few shots in my gallery of wild catcus in the San Miguel area.