A few lines (including Royal Caribbean, Celebrity, Princess and Norwegian) allow 18- to 20-year-olds to drink alcohol in international waters if their parent or adult guardian signs a waiver form.

While most U.S.-based cruise lines like Carnival and Royal Caribbean enforce a strict minimum drinking age of 21, several lines allow 18-year-olds to drink on specific itineraries. Virgin Voyages and Costa Cruises generally have a drinking age of 18 on sailings outside of the United States. Norwegian Cruise Line (NCL) and Oceania Cruises permit guests aged 18–20 to purchase and consume beer and wine in international waters (3 miles out) with a signed parental waiver, except on Alaska or New England routes. Disney Cruise Line also lowers the age to 18 for cruises departing from European ports or Australia where local laws allow. For most major lines, if the ship departs from a U.S. port, the age remains 21 for the entire duration of the cruise to remain compliant with U.S. maritime and insurance standards.