To use the seas, a boat needs to have the permission to fly the flag of a country. That country is its flag state. The right to fly a country’s flag is often granted through boat registration but that isn’t always the case. The boat must comply with the applicable laws of the flag state.
If a boat is used in a country other than its flag state, it must comply with the regulations of that country (the coastal state) in addition to those of its flag state.
To understand the requirements for evidence of competence it’s necessary to know what is required by the flag state and what is required by the coastal state.
Find out more about boats flagged / registered in the UK.