Irma pushes north and is causing problems in Charleston, S.C.

High winds felled trees and severed service lines in Georgia and South Carolina on Monday, knocking out power for more than 900,000 electricity customers in the two states.

A tropical storm warning was issued for all of Georgia’s Atlantic coast and most of the South Carolina coast. Some of the worst flooding occurred in Charleston, where knee-high floodwaters coursed through the streets — high enough for some residents to navigate by kayak.

The National Weather Service issued a flash flood emergency for Charleston County, and said that parts of the Charleston peninsula, which contains the city’s historic core, were being closed down.

In an interview Monday afternoon, Mayor John Tecklenburg said that the city had been hit with a four-foot storm surge, leaving parts of the peninsula looking as if they had merged with the Ashley River.

“It sounds kind of counterintuitive that we’d have that, because the center of the storm is over 200 miles away in western Georgia, and here we are over on the coast of South Carolina,” he said. “But just if you looked at the bigger weather map and saw the counterclockwise rotation of Irma, juxtaposed with a clockwise high-pressure rotation over the Atlantic, Charleston was like in the pincer of those two motions that has driven wind and hurricane bands almost directly into our city.”

Mr. Tecklenburg said that the flooding was even worse than last year’s Hurricane Matthew, which inundated the city in October, in great part because Matthew arrived at low tide, whereas Irma’s effect came at high tide.

Farther inland, concerns about serious damage remained high, even as the storm’s power diminished somewhat.

In Atlanta, the winds whipping through the leaves created a sound like an angry sea breaking on a shoreline, and trees crashed into residences and onto roadways. The city’s public school system canceled classes through Tuesday, and Delta airlines, based in Atlanta, canceled about 900 flights Monday, noting a special concern about strong north-south crosswinds at Hartsfield-Jackson Atlanta International Airport, which bills itself as the busiest in the world.

The forecast in Alabama was somewhat milder, though a tropical storm warning was in effect for much of the state’s eastern half.

Jacksonville is inundated, while Tampa is ‘looking good’

“We need you to heed our warnings,” Mayor Lenny Curry of Jacksonville said on Monday, explaining that high tides would raise river waters up to 6 feet above their normal levels and cause additional flooding.

The mayor urged residents to avoid drawing on city resources except for in emergencies, but said people who needed rescuing should raise a white flag to draw the authorities’ attention.

Jacksonville was facing a “trifecta” of water-related threats, city officials said: storm surge, heavy rainfall over the weekend and Monday’s rising tides. “This is potentially a weeklong event with water and the tides coming and going,” Mr. Curry said.

In Tampa, Mayor Bob Buckhorn, who on Sunday warned residents that the city was about to get “punched in the face,” said on Monday that the city had been spared the storm’s worst.

“It’s looking good,” Mr. Buckhorn said. “The first blush is that not only did we dodge a bullet, but we survived pretty well. Not a lot of flooding. Tree removal, debris — don’t want to say it’s negligible, but it’s manageable.”

The city was again spared from a direct hit by a hurricane, as has been its good fortune for more than 90 years running. How? “Because we live good lives, because we only get drunk once in a while,” Mr. Buckhorn joked. “No, I don’t have an answer for that.”

In St. Petersburg, tree limbs littered lawns and minor debris had blown onto roads but was not stopping traffic. In Orlando, officials said the city had weathered the storm without major damage.

The governor surveys the Florida Keys from the air

Governor Scott inspected the Florida Keys on Monday from a Coast Guard aircraft, saying the flight revealed extensive flood damage, boats washed ashore and trailer parks filled with overturned homes. “My heart goes out to the people in the Keys,” he said.

Mr. Scott also flew over the state’s Gulf Coast and reported there was less damage than had been feared. “We saw the remnants of the storm surge along the west coast, but I didn’t see the damage I thought I would see,” he said.

“It’s not as bad as we thought the storm surge would do,” he added.

He asked for patience as the state deals with a long recovery. “I know for our entire state, but especially the Keys, it’s going to be a long road,” he said.

Here’s how officials tried to help people on Marco Island

Picking their way through ruined and waterlogged streets, rescue officials searched on Monday through neighborhoods on Marco Island, checking in on people who stayed behind during the storm, which made landfall there on Sunday.

“As soon as it was safe, we went outside and immediately began as the storm was coming at us and during the eyewall,” Capt. Dave Baer of the Marco Island Police Department said in an interview on Monday.

Officials were also assessing tens of thousands of homes and condominiums on the island, where an estimated 30 percent of the 20,000 permanent residents did not evacuate, Captain Baer said.

There were no deaths and no serious injuries, he said. The rescues were all fairly similar — people stuck in cars as they tried to evacuate or in houses that had some sort of structural collapse.

The storm surge had receded by daybreak. “Now all our streets are dry,” said Larry Honig, chairman of the City Council. “We are without power and water, but presumably we will be restored in a couple of days. There is minimal structural damage. I know of no lost homes.”