When an aircraft is flying over international waters (high seas) it should follow the ICAO Annex 2 rules of the air. On the other hand, when an aircraft is flying over the territory of a State, it should follow the rules of the air of that State unless they are similar to the ICAO Annex 2 rules.

This question is similar to 102211:

• communication failure occurs over US territory → follow US rules of the air
• communication failure occurs over international waters (high seas) → follow ICAO Annex 2 rules of the air

High seas. All parts of the mass of saltwater surrounding the globe that are not part of the territorial sea or internal waters of a state.

SERA.1001 General

(a) For flight over the high seas, the rules specified in Annex 2 to the Chicago Convention shall apply without exception. For the purposes of continuity and seamless operation of air traffic services in particular within Functional Airspace Blocks, the provisions of Annex 11 to the Chicago Convention may be applied in airspace over high seas in a manner that is consistent with how those provisions are applied over the territory of the member States. This shall be without prejudice to the operations of State Aircraft under Article 3 of the Chicago Convention. This shall also be without prejudice to the responsibilities of Member States to ensure that aircraft operations within the Flight Information Regions within which they are responsible for the provision of air traffic services in accordance with ICAO regional air navigation agreements are undertaken in a safe, expeditious and efficient manner.

(b) For those parts of the high seas where a Member State has accepted, pursuant to an ICAO regional air navigation agreement, the responsibility of providing air traffic services, the Member State shall designate the ATS provider for providing those services.