What is the drinking age on a cruise ship?

The whole 'drinking underage in international waters' is incorrect. Most cruise lines the drinking age is 21, no matter where you cruise. My husband and I took a Med cruise out of Southampton, England with Princess and the drinking age was 21 the entire time, even though the drinking age in the UK is 18. So I doubt you'll be able to drink.

First, I don't think there are any Canadian cruise companies, so your question and premise are moot.

The minimum drinking age on all ships departing US ports is age 21, and that does not change when in international waters or when the ship is in port in a jurisdiction with a lower drinking age on shore. Those under age 21 may drink on shore where the legal drinking age is under 21 (most of the Caribbean).

Some cruise lines allow those 18 and older to drink on board under very limited circumstances -- the ship departs a non-US port such as a port in Central or South America or some European countries; the parent signs special permission AND is on the cruise with the minor; only beer and wine are allowed. Not all cruise lines allow this.

Well, there are no Canadian cruise ships however looking at it from that point while in Canadian or International waters you would have to be 19 to legally drink. If you entered US waters technically you will then have to be 21 to be able to drink on board the ship. Depending on the length of time the ship is in the waters the age limit may not change. To use a more real example, people travelling on a Caribbean cruise can drink at age 18 although the cruise lines require a waiver signed by the parents. However taking the same cruise ship on an Alaskan cruise the passengers must be 21 to drink and no waivers are available.

I'm fairly sure that when you are on the ship the laws follow the laws of that country, so even though you may pass the US if you are on the ship then that does not matter.

I'm going on a cruise (an American line), and so the age on the ship is 21 but once I get off the ship in Canada, their laws apply so I can do what I want.

Hope this helps!

You are in international waters so no national law applies. HOWEVER all ships have a policy. It is usually 21.

Forget about where the ship is. It is 21 wherever the ship might be (if a US company)

If on a European company ship, sailing in Europe that might be different.

Just ask the bartender at the bar when you get on the ship.