Relief operations are under way and engineers are working to restore power, but many areas remain stranded.
The islands of the Florida Keys and western parts of the US state bore the brunt of the category-four hurricane.
Irma hit Florida on Sunday, before weakening to a tropical storm as it moved north on Monday.
Media reports link at least four deaths to the storm in Florida. Last week it killed at least 37 people in Caribbean islands.
White House Homeland Security Adviser Tom Bossert said it will be some time before residents in the Florida Keys are able to return to their homes.
“I would expect that the Keys are not fit for re-entry for regular citizenry for weeks”, he said.
Speaking as he went on an aerial tour of the Keys, Florida Governor Rick Scott said: “Power lines are down throughout the state. We’ve got roads that are impassable, so everybody’s got to be patient as we work through this.”
The Keys are cut off from the mainland, as the 42 bridges that link them are being assessed for damage. Reports say that 10,000 people decided to ride out the storm
How big will the disaster response be?
Although Miami was spared the worst, large parts of the city are under water. Winds have snapped power lines and 72% of homes there are without electricity, officials say.
On the west coast of Florida, drone footage from Naples, a town on the coast of the Gulf of Mexico about 125 miles (200km) to the north-west, shows rows of shattered suburban homes on streets under water.
President Donald Trump has released emergency federal aid for Florida, describing the hurricane as a “big monster”.
Culled from BBC