I have the same question, the Googling of which led me to this discussion. I found "national open bar" in the description of the Playa Mia Grand Beach Park in the NCL excursion "Mexican Cuisine Workshop & Tasting":
> Your tour includes round-trip transportation, a fully-equipped cooking station, national open bar, entrance to the park, and access to other top-notch services, facilities, and exciting activities including lounge chairs, beach umbrellas, a large shopping area, underwater Mayan city, organized games, kayaks, pedal boats, and much more!
My Googling also led me to Cancun Sailing: What is the difference between the National Open Bar and the Premium Open Bar?:
[quote]
National Open Bar:
Bacardi Carta Blanca Rum
Jose Cuervo Tequila
Smirnoff Vodka
Sol or Superior Beer
Mc Andrews Whisky
Soft Drinks: Pepsi, grape juice, and purified water
Premium Open Bar:
Matusalem Rum
Tequila Jose Cuervo Especial Reposado
Smirnoff or Absolut Vodka
Red Label Whisky
XX Lager beer
White/red wine Concha y Toro
Soda: Coca-Cola, mineral water.
Juice: orange, pineapple, tomato and cranberries
Bottled water
[/quote]