A debate was ensuing at work about consuming your own booze onboard a plane.
The FAA can fine you several thousand dollars in the US if you did this, so naturally this led to people theorising ways around this.
The top theory was that once you were over international waters, it would be permitted. I'm fairly sure you come under the laws of the company operating the aircraft, but don't have evidence for it.
So - is it legal to consume your own alcohol onboard over international waters?
It depends primarily on the airline policy. The only airline I believe is Royal Brunei Airlines which allows consumption of personal alcohol on-board because it does not serve any on the aircraft.
Any airline which serves alcohol on-board typically does not allow consumption of personal alcohol. I suspect the reason for this is to maintain decorum during the flight and prevent people from drinking too much which would compromise the safety of other passengers including the intoxicated person in tense situations.
As mentioned in the question, the FAA prohibits alcohol which is not served by the certificate holder operating the aircraft. So, this rules out the United States.
Most airlines don't provide any specific information about it in their General Conditions of International Carriage for Passenger and Baggage. If you want to look up for a specific airline, the Conduct Aboard the Aircraft (Article 11,12 or 15 typically) can provide information regarding the same.
One example we can talk about is airlines which do not serve alcohol on their flights:
Saudi Arabian Airlines (Saudia) does not allow it in any form
Saudi Arabian Airlines does not serve or permit passengers to carry or drink alcoholic beverages on its flights.
Royal Brunei Airlines doesn't serve alcohol but allows you to carry some and consume on board (does not specify domestic or international waters). It also does not mention anything regarding this in their Carriage Conditions.
Pakistan International Airlines does not allow it in any form
You are not allowed to consume alcohol aboard an aircraft (whether purchased as duty free or otherwise obtained)
Another example we must talk about is European airlines which do serve alcohol:
Swiss Airways serves alcohol but does not allow passengers to consume their own alcohol.
It is forbidden to consume any alcohol which you may have personally brought on board.
Condor also serves alcohol but does not allow personal alcohol.
Passengers are not allowed to consume the alcoholic beverages they bring on board during the flight, including duty-free items purchased at the airport or on the plane.
KLM follows the same policy as the others
15.4 The Carrier may limit or prohibit the consumption of alcohol on board the aircraft. Consumption of any alcoholic beverages carried into the aircraft by Passengers or consumption of any duty free product bought on board the aircraft is prohibited.
So does Virgin Atlantic
For safety reasons, consumption of alcohol by passengers is restricted to alcohol served by us, with the result that alcohol purchased by passengers (whether as duty free purchases at airports or aboard aircraft or other purchases) cannot be consumed aboard our aircraft.
Now, airlines in Asia which also serve alcohol:
Air India doesn't say it specifically but it implies that personal alcohol consumption is not allowed
Passengers are also permitted to consume their own snacks and non-alcoholic drinks on board.
China Eastern says it quite specifically
Except for the alcoholic drinks provided by CEAIR flights, Passengers shall not have any other alcoholic drinks on board the aircraft.
Ukraine International Airlines does not either
Drinking of any alcoholic drinks on board an aircraft is permitted only in amounts offered by the airline.
Gulf Air has the same policy as the others
You are not allowed to consume alcohol aboard an aircraft (whether purchased as duty free from us or someone else or otherwise obtained) unless it has been served to you by us. We have the right, at any time and for any reason, to refuse to serve you alcohol or to withdraw alcohol which has been served to you.
