Gov. Rick Scott refused to budge on changing the deadline, but a federal judge ordered an extension Wednesday morning.
The storm was "hugging" Florida's coast, bringing pounding rain and wind along the state's eastern shores.
More than 2.5 million people have been ordered to evacuate homes from Florida to South Carolina.
Officials have ordered evacuations, shuttered schools and emphasized the danger posed by the storm.
The storm began to weaken after landfall but still packed strong winds and flooding rain.
More than 20,000 people were rescued from their homes as flooding swept across the southern part of the state, officials said.
Authorities said the boy was playing in a back yard in Ravenswood, W.Va., when he fell or jumped into fast-moving water.
Forecasts say parts of Massachusetts could get up to 18 inches of snow.
Authorities in cities and states along the East Coast pleaded with people to stay off the roads and warned of worsening conditions Saturday.