They do sell bottled water in the 10+ gallon jugs like we have here in the offices in the USA. My wife's family gets that for me when I visit Mexico during the holidays. You might want to go that route with a water delivery vendor and just pump
it into your boat. The bottled jugs from a reputable company should work fine. I have not had any issues with it during any of my visits. Do try the Coke or Pepsi local distributors as well as suggested above. In Mexico they do sell large container bottled water as well. At least they do in my wife's small town.