Too much sun, too much alcohol, not enough water, and foods you aren't used to eating can all add up to stomach problems. I think a lot of people that have any kind of tummy troubles in Mexico automatically blame it on monteczuma's, but, if it happens anywhere else on vacation... and it does... or at home after a hot summer weekend spend out in the sun eating and drinking... and it does... they realistically blame their own choices and actions.
I have a cheesecloth stomach and any tropical destinations I go to I stay away from tap water, buffets, most fried foods, dairy if it's really hot & refrigeration can be iffy, and I keep well hydrated with water and wait til the very late afternoon to have any alcohol (and don't overdo it, keep drinking water too). Eating fresh, local foods that are prepared to order seems to work best for me.
In most tourist destination these days, ice is made from purified water so no worries with ice unless you're in a very remote place.... but then you probably won't even get ice. Produce is also washed in solutions like Microdyne (or other iodine based produce wash products). No restaurant or resort wants sick tourists.
The few times over the last 20 or so years that I have had a problem in Mexico, a bowl or two of a really good sopa de lima does wonders. (Happens to be one of my all time favorite soups too.)
I personally don't like the idea of taking pepto or any OTC gastro stuff preventatively. The amount you'd have to take to make it 75% effective is 1oz every 1/2 hr until you've had 8 doses a day (
Monteczuma's Revenvge)