The United States Navy dispatched the USS Iwo Jima, USS New York and the aircraft carrier Abraham Lincoln to the Florida Keys help with search and rescue and other relief efforts.
President Trump sent the aircraft carrier and other Navy ships on Monday as a flyover of the hurricane-battered Keys yielded what the governor said were scenes of devastation. This gives all the Keys a fully functional airport - instantly - and numerous helicopters which will speed rescue efforts.
"I just hope everyone survived," Gov. 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 Sunday morning with 130 mph (209 kph) winds. How many people in the dangerously exposed, low-lying islands defied evacuation orders and stayed behind, was unclear.
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.
"HELP IS ON THE WAY," they promised on Facebook.
The Keys are linked by 42 bridges that have to be checked for safety before motorists can be allowed in, officials said. The governor said the route also needs to be cleared of debris and sand, but should be usable fairly quickly.
Over the next two days, Irma is expected to push to the northwest, into Alabama, Mississippi and Tennessee.
People in the heavily populated Tampa-St. Petersburg area had braced for the first direct hit from a major hurricane since 1921. But by the time Irma arrived in the middle of the night Monday, its winds were down to 100 mph (161 kph) or less.
"When that sun came out this morning and the damage was minimal, it became a good day," said Tampa Mayor Bob Buckhorn.