The Drink Tax like others have said will be applied when still in US Waters. So sailing out of Miami, Port Canaveral, and Port Everglades you will be taxed until you hit International Waters. Doesn't take long to get out there but if you board at noon anticipate about 70 cents for each drink.
Now sailing out of new orleans it take 6 to 8 hours to hit international waters, noon till around midnight.