Meteorologists said storm surge was measured at more than 4 feet in some areas.
"We are seeing impacts right now from St. Augustine to Jacksonville Beach. Unfortunately, this is going to continue through the night," Gov. Rick Scott said.
The storm, which is blamed for four deaths in Florida, sparked flash-flood concerns. The National Weather Service issued a warning for parts of Nassau County, north of Jacksonville.
"The combination of a dangerous storm surge, the tide and large and destructive waves will cause normally dry areas near the coast to be flooded by rising waters moving inland from the shoreline," the National Hurricane Center said.
Special concern surrounded Jacksonville's St. Johns River, which could be overwhelmed by water pushed into it by the storm. The hurricane center said a tide gauge reported storm surge of 4.28 feet.
After killing hundreds in Haiti and other Caribbean nations, Matthew's frightening power led to the deaths of four people in Florida. The list included a woman in her 60s in Volusia County, hit by a falling tree, and an 82-year-old man and a woman in St. Lucie County, officials said. The latter two had medical emergencies and responders were unable to reach them in time because of hazardous weather.
A woman in northeast Florida died after a tree fell on her camper trailer, the Putnam County Sheriff's Office said.
The Category 2 hurricane, while losing some of its wind speed, left more than 1 million people without power in Florida as it spent the day plowing north just off the state's east coast. Late Friday, Matthew was hovering off the coast of Georgia and headed toward South Carolina, where officials issued warnings about the possibility of treacherous storm surge and up to 15 inches of rain.
Matthew is expected to arrive near or over the shores of South Carolina by Saturday and over to North Carolina by Saturday evening.
Winds were already blustery in Hilton Head, South Carolina, with one gust measured at 63 mph.
It's not as if Matthew -- with winds of 105 mph -- won't cause further damage with its powerful winds.
"Just because the center of circulation is offshore doesn't mean you can't be the center of action (along the coast)," National Hurricane Center Director Rick Knabb said. "It's going to get a lot worse before it (has) a chance of getting better."
Here's what you need to know:
• As of 11 p.m. ET, Matthew's center was over the Atlantic, about 70 miles south-southeast of Savannah, Georgia, the National Hurricane Center said. Hurricane-force winds extended outward up to 60 miles from that center. It was moving north at 12 mph.
• A total of 1.1 million customers in Florida were without power.
• The National Weather Service warned that some places hit by Matthew could be uninhabitable for "weeks or months."
• The storm has killed at least 300 people
in three Caribbean countries. The majority died in Haiti, said Civil Protection Service spokesman Joseph Edgard Celestin.
Georgia, South Carolina and North Carolina
As northeastern Florida braced for impact, coastal communities in Georgia, South Carolina and North Carolina also were on notice. The storm's center could be near or over the coasts of Georgia and South Carolina on Saturday, the hurricane center said.
• Glynn County officials wrote on Facebook that "Conditions have deteriorated to a point that persons remaining in (the area near Brunswick) are advised to shelter in place for the remainder of the storm."
• Maj. Tommy Tillman of the Chatham County Sheriff's Office said the road to Tybee Island is closed.
• In Savannah, Mayor Eddie DeLoach warned those who stay that they'd be on their own.
• Gov. Nathan Deal has ordered evacuations for all counties east of Interstate 95. Deal has activated 1,000 National Guard troops.
• Four counties and one city in South Carolina will be under nighttime curfews until further notice, said state Emergency Management Division spokesman Derrec Becker. Dorchester, Beaufort, Jasper and Williamsburg counties will be under curfew. Charleston's curfew starts at midnight. The curfews end each morning, at different times.
• Gov. Nikki Haley warned residents who didn't evacuate to go to a shelter. A major storm surge of 8 feet or more is approaching low-lying areas in the state, including Charleston.
• Although 310,000 people have evacuated the area, Haley says that's not enough. Officials in some areas are going door to door, urging people to leave. Police in Pawleys Island asked residents who decided to stay in spite of the evacuation orders to sign a waiver and list their next of kin
, according to CNN affiliate WBTW
• Gov. Pat McCrory said the forecast had changed, to North Carolina's detriment. "What we feared is now happening in North Carolina. The (hurricane forecast) model has changed dramatically," he said. "The immediate concern is life-threatening rain and water (from storm surge)."
• Officials are concerned that areas -- such as Fayetteville, home to the Army's Fort Bragg -- that were recently flooded will see more rain from Matthew.