From the medicare.gov website:

Travel (when you need health care outside the U.S.)
How often is it covered?
In general, health care you get while traveling outside the U.S. isn't covered. The 50 states, the District of Columbia, Puerto Rico, the U.S. Virgin Islands, Guam, the Northern Mariana Islands, and American Samoa are considered part of the U.S. **
Medicare may pay for inpatient hospital, doctor, ambulance services, or dialysis you get in a foreign country in these rare cases:
You're in the U.S. when a medical emergency occurs, and the foreign hospital is closer than the nearest U.S. hospital that can treat your medical condition.
You're traveling through Canada without unreasonable delay by the most direct route between Alaska and another state when a medical emergency occurs, and the Canadian hospital is closer than the nearest U.S. hospital that can treat the emergency.
You live in the U.S. and the foreign hospital is closer to your home than the nearest U.S. hospital that can treat your medical condition, regardless of whether an emergency exists.
In some cases, Medicare*may cover medically necessary health care services you get on board a ship within the territorial waters adjoining the land areas of the U.S. Medicare won't pay for health care services you get when a ship is more than 6 hours away from a U.S. port.
Medicare drug plans don't cover prescription drugs you buy outside the U.S.
Medicare supplement insurance (Medigap) policies may cover you when you travel outside the U.S.