"I just hope everyone survived," Governor Rick Scott said.
He said boats were cast ashore, water, sewers and electricity were knocked out, and "I don't think I saw one trailer park where almost everything wasn't overturned." Authorities also struggled to clear the single highway connecting the string of islands to the mainland.
The Keys felt Irma's full fury when the storm blew ashore as a Category 4 hurricane on Sunday morning with 209km/h winds. How many people in the dangerously exposed, low-lying islands defied evacuation orders and stayed behind was unclear.
As Irma weakened into a tropical storm and finally left Florida on Monday after a run up the entire 640km of the state, the full scale of its destruction was still unknown, in part because of cutoff communications and blocked roads. On Monday night, the storm had weakened to a tropical depression near Columbus, Georgia.
Six deaths in Florida have been blamed on Irma, along with three in Georgia and one in South Carolina. At least 35 people were killed in the Caribbean.
Emergency managers in the islands declared on Monday "the Keys are not open for business" and warned that there was no fuel, electricity, running water or cell service and that supplies were low and anxiety high.
Statewide, an estimated 13 million people, or two-thirds of Florida's population, remained without power. That's more than the population of New York and Los Angeles combined. Officials warned it could take weeks for electricity to be restored to everyone.
More than 180 000 people huddled in shelters in the Sunshine State.
The governor said it was way too early to put a dollar estimate on the damage.