Continued weakening was forecast and the U.S. National Hurricane Center said Irma was expected to become a tropical storm over northern Florida or southern Georgia later in the day Monday.
Warnings of risky storm surges remained in effect through vast swaths of peninsular Florida, where more than six million people had been ordered to flee Irma in one of the biggest evacuations in United States history.
Irma made its second US landfall at Marco Island at about 3:30 p.m. One stretch of the bay was six feet below normal tide. No injuries have been reported. Officials warned it could take weeks for electricity to be restored to everyone. Miami Beach barred outsiders from the island.
Monroe County Administrator Roman Gastesi said relief will arrive on a C-130 military plane Monday morning at the Key West International Airport.
In one of the largest US evacuations, almost 7 million people in the Southeast were warned to seek shelter, including 6.4 million in Florida alone.
Irma, meanwhile, was making its way north after making landfall on the southwestern coast of Florida.
But then Irma made a more pronounced westward shift - the result of what meteorologists said was an atmospheric tug-of-war between weather systems that nudged Irma and determined when it made its crucial right turn into Florida. Evacuees Of The Keys shared school closure notices, videos of destruction, and manyposts from friends and relatives searching for loved ones.