In the United States, you need to be 12 miles off the coast, whereas in the UK, you need to have embarked on a journey into international waters. If you're not sure about the local laws in your country of departure, make sure to check beforehand. Gambling laws are subject to change and not always straightforward.
Offshore gambling encompasses any gambling that takes places at online sportsbooks, casinos, and horse racing sites operating outside of US borders. These offshore sites do not hold licenses to operate in US jurisdictions and are therefore unregulated.