Dozens more were rescued from a collapsed motel.
North Carolina utilities estimated that as many as 2.5 million state residents could be left without power, the state's Department of Public Safety said.
On Friday, coastal streets in the Carolinas flowed with frothy ocean water, pieces of torn-apart buildings flew through the air, and at least 525,000 homes and businesses were without power, almost all of them in North Carolina.
Some parts of North Carolina have already seen surges as high as 10ft in places.
A mother and infant were killed in Wilmington, North Carolina, when a tree fell on their home.
Two died in Lenoir County - a 78-year-old man was electrocuted while trying to connect extension cables, and another was knocked down by heavy winds.
One resident, restaurant owner Tom Ballance, told the Associated Press he now thinks he should have evacuated.
Tropical storm conditions will continue along the coast within the tropical storm warning area and also well inland across portions of SC and North Carolina Saturday. It blew ashore along a mostly boarded-up, emptied-out stretch of coastline.
Forecasters believe the biggest danger right now is the water, not the wind. With Florence, "we're looking at the same amount of rainfall in three days".
The flooding soon spread into SC, swamping places like North Myrtle Beach, in a resort area known for its white sands and multitude of golf courses. To the south, storm surge pushed water levels at Johnny Mercer Pier at Wrightsville Beach, where Florence made landfall at 7:15 a.m. with 90 miles per hour winds, to more than 8.5 feet.
Florence was a Category 3 hurricane on the five-step Saffir-Simpson scale with 120-mph winds on Thursday.
The National Hurricane Center says that "catastrophic" freshwater flooding is expected over portions of the Carolinas as Hurricane Florence inches closer to the U.S. East Coast. Florence was one of two major storms threatening millions of people on opposite sides of the world. Forecasters say it could become a depression by Saturday night.
Evacuation warnings were issued for 1.7 million people in the region.
Authorities warned, too, of risks of mudslides and environmental disasters from floodwaters washing over industrial waste sites and hog farms.
Florence's eye made landfall over the Wrightsville Beach water tower about 7:15 a.m. Friday, according to the National Weather Service (NWS) office in Wilmington.
"Rivers are rising to unsafe levels, and the relentless rains will continue through the weekend", North Carolina Gov. Roy Cooper warned on Friday.
North Carolina alone is forecast to get 9.6 trillion gallons (36 trillion liters), enough to cover the Tar Heel state to a depth of about 10 inches (25 centimeters).
On Friday, coastal streets in the Carolinas flowed with frothy ocean water, and pieces of torn-apart buildings flew through the air.
Current wind gusts are shown below. These rains are expected to produce "prolonged, significant river flooding". "If the number seems really insane high, you always have to have some suspicion until we can actually get out and verify it". "We're going to have to have patience, we're going to have to be careful and we're going to have to deal with a lot of water".
Zaytoun said he didn't leave his home because he knew he wouldn't have been able to get back to it once the flooding began.
Heavy rain and flooding are expected to continue at least through the weekend, and local officials said hundreds in areas hit by Florence still need rescue.