Flooding across the Carolinas was expected to worsen over the next couple of days, forecasters said.
Officers say it appears the water knocked the man's vehicle off the roadway.
At Horry County's emergency operations center, South Carolina Gov. Henry McMaster said the storm had likely been the worst disaster in the state's history, but warned it wasn't over yet.
Florence dropped as much as 23 inches of rain in some parts of the state.
With muddy river water still washing over entire communities on Friday, eight days after Hurricane Florence slammed into land with almost 3 feet of rain, new evacuation orders forced residents to flee to higher ground amid a sprawling disaster that's beginning to feel like it will never end.
North Carolina Gov. Roy Cooper says the state is still tallying its storm damage, but says it will be in the billions. The barrier island community of Emerald Isle started feeling the impacts of Florence more than a day before the hurricane made landfall, and a US government water monitoring gauge ultimately recorded water levels over 6 feet (2 meters) above normal there.
The Latest on the effects of Hurricane Florence.
State-owned utility Santee Cooper in SC is placing an inflatable dam around a coal ash pond near Conway, saying the extra 2.5 feet should be enough to keep floodwaters out.
In Lenoir County, North Carolina, where the rising Neuse River has flooded some roads, emergency medical workers have been running a "mobile disaster hospital" to provide urgent care to those residents who were cut off from the nearest hospital.
President Trump visited both North and SC on Wednesday.More news: With Kavanaugh allegations, Democrats warn of repeating mistakes of Anita Hill hearings
More news: Hurricane Florence projected path: When will Florence track hit Carolinas?
More news: Amazon tipped to launch 8 Alexa smart home and vehicle gadgets
A ninth person has died in SC because of Hurricane Florence.
"Everybody's closed, so we're hoping we can at least get a tarp here, maybe two", Karen Foster told ABC News.
Cooper said Thursday there were 31 storm-related deaths statewide.
"I'm so sad just thinking about all the work we put in".
Thousands of people evacuated their homes, at least 37 people died, and millions lost power.
More than three dozen flood gauges in North and SC showed flooding, according to the National Weather Service.
"They didn't clean the ditches", she said.
Allison Cavenaugh and her husband, who own a home and a poultry farm with 80,000 chickens in Wallace, North Carolina, returned home for the first time this week on a boat just to find out that their chickens have drowned.
When the water eventually recedes, North Carolina residents will have to not only deal with the usual clean-up challenges but also added health-risks caused by its key industries: pig farming and coal power generation. Many people in the crowd smiled and raised their phones to record the moment.
In North Carolina, a familiar story was unfolding as many places that flooded in Hurricane Matthew in 2016 were once again inundated.
In South Carolina, farmers face damage to cotton, peanut and hemp crops, the state said.