I have been to many places in Mexico- at least a dozen on both sides. I have never gotten sick from water or food anywhere. Keep in mind that tourism is the major revenue in these spots so the last thing the locals want are sick tourists who will not want to return. Enjoy a Margarita and have fun!
The chances of picking up "Montezuma's Revenge" in the major restaurants and bars in the cruise ports is very low. If you purchase foods from street vendors or street markets then you need to be much more careful.
For walking around I use bottled water and purchase sodas.
Many major American brands of liquor, beer, and wine coolers are available in the bars but I suggest that you be a bit adventurous and try a local beer or beverage. You are only there for a short time so why not make the most of it.
Have a great time.
__________________ Cruisemates Community Leader/Moderator
"There is a great difference between being well traveled and just having been to many places." ~Me
You will not have any problems. The only thing you need to be careful of would be any street vendors selling food or unsealed refreshments or the little mom & pop eating places. The water in the areas that the ships are going to is pretty safe as they collect the rain and it is filtered. All the major places are as safe as you could want.