Evacuees have begun to return to the storm-ravaged islands of Key Largo, Tavernier and Islamorada to find their homes ripped apart and their businesses coated in seaweed.
The death toll from Irma, previously ranked as one of the most powerful Atlantic storms on record and the second major hurricane to strike the US mainland this season, has climbed to more than five dozen.
Of those, 43 were killed in the Caribbean and at least 18 in the southeastern United States.
Twelve Irma-related deaths were confirmed by Florida emergency management officials, while authorities in Georgia and South Carolina each reported three deaths from the storm and its immediate aftermath.
Destruction was widespread in the Keys, a resort island chain stretching southwest from the tip of the Florida Peninsula into the Gulf of Mexico and connected by a single, narrow highway and a series of bridges along a route of nearly 100 miles (160km).
Federal Emergency Management Agency administrator Brock Long said preliminary estimates suggested 25 per cent of the homes in the Keys were destroyed and 65 per cent sustained major damage.
"Basically, every house in the Keys was impacted," he said.
Although the Keys are studded with mansions and beachfront resorts, about 13 per cent of the people live in poverty and could face further hardships as the cleanup begins.
"People who bag your groceries when you're on vacation — the bus drivers, hotel cleaners, cooks and dishwashers — they're already living beyond paycheck to paycheck," Stephanie Kaple, who runs an organisation helping the homeless in the Keys, told the Associated Press.
Glimpses of Irma's economic toll are beginning to emerge, with Florida saying 31 state agencies had already amassed nearly $250 million in preparation and recovery expenses.
The number of people without electricity in the steamy late-summer heat dropped to 9.5 million — just under half of Florida's population. Utility officials warned it could take 10 days or more for power to be fully restored.
About 110,000 people remained in shelters across the state.
Irma was downgraded to a tropical depression late on Monday as it drifted into Alabama and was likely to dissipate further on Tuesday evening, the National Hurricane Centre said.
Additional reporting by agencies