David and I go to Mexico often. We have two retired doctor friends that live in Puerto Vallarta. Here are their suggestions which we have had very good luck following:
> Get a five day or six day prescription for "Cipro" from your doctor and keep it in your suitcase just in case of severe nausea and/or diarreah. Begin taking this medicine when symptoms begin. Continue taking pills twice daily until the five or six day course is finished. You will feel much better 24 hours after you begin taking this antibiotic..
Cipro is also available over the counter in Mexico and is packaged as Ciprocina. The generic name is Ciprophloxocin.
> As a preventative, take 2 Pepto Bismo tablets prophylactically each morning before going ashore. We have followed this suggestion and NEVER had a problem, remembering that we are often in Mexico about a Month or so each year. We eat in all restaurants that appear clean and do not avoid any kinds of foods. We do drink bottled water but also brush our teeth with the local water.
Don't worry. You will have a great time!