Folks enjoy the beach next to a washed-up sailboat from Hurricane Irma at Miami Marine Stadium on Monday. (Ricky Carioti/The Washington Post)

As Hurricane Irma dissipated into a tropical storm on Monday, Florida's residents emerged to streets littered with debris and downed trees while nearly two-thirds of the state was left without electricity.

The once-powerful storm left trailer homes sliced open like ripe melons, boats tossed upside down on roadways and centuries-old trees strewn across power lines. As it trailed off on Monday, Irma’s rains caused floodwaters to rise from Jacksonville, Fla., to Charleston, S.C., continuing to impact a massive area of the American southeast.

But it could have been much worse.

That was the grateful mantra on the lips of many on Monday, even as an estimated 12 million Floridians prepared for a dark night without air conditioning in the muggy post-storm swelter. Though there was significant property damage in the Florida Keys and in some parts of southwest Florida, officials said it was remarkable that so far they are investigating just a small number of fatalities that came as the storm made landfall. It was unclear how many were directly related to the storm.

The lack of electricity across most of South Florida was the most pressing and crippling problem. Millions could remain in the dark for days or even weeks as utility companies struggle to navigate impassable roads and floodwaters to slowly restore power.

But in the face of cataclysmic warnings and worries — including a mass exodus from Florida’s most-populous area — Irma largely spared many of the major cities predicted to be in its path. Some, including Tampa and Orlando, escaped relatively unscathed. Others, such as Jacksonville, experienced unlikely — and record-breaking — effects.

Waters in Jacksonville, in the state’s far northeast, sent residents scrambling to the top floors of their houses Monday morning. The St. Johns River, which cuts through the city, overflowed its banks, flooding bridges and streets.

Rescuers used boats, water scooters and even surfboards to get to residents surprised by the rising waters, said Kimberly Morgan, a spokeswoman for the Clay County emergency center. “You have to get creative in a situation like this,” she said.

Morgan said that evacuation shelters, which already held 700 people before Monday, we’re expected to fill up even more. “We don’t think we’re going to see the end of this until Friday,” she said.

Scores of power lines went down as a result of Hurricane Irma’s winds along Corkscrew Road near Estero, Fla. (Michael S. Williamson/The Washington Post)

Authorities warned that it was not yet safe for evacuated residents to return to their homes in many areas of Florida, the threat of floods still looming as rivers swell with rainwater and storm surges continue to send rising ocean waters into towns, especially in northern Florida. And state officials warned that another approaching storm, Hurricane Jose, is pushing still more water toward the northern part of the state.

Gov. Rick Scott (R) called the flooding in Jacksonville “historic” — officials said the city could end up with four feet of standing water — and he warned the many residents still stuck in the dark that “it’s going to take us a long time to get the power back up.”

Marilyn Miller awoke in St. Petersburg at 1:30 a.m. Monday to a pitch-black house. A native Floridian, Miller was expecting the outages and has even gotten used to them after enduring years of tropical storms.

What she didn’t expect, she said, was the possibility that the blackout could last for days.

As neighbor after neighbor on her block tried to call Duke Energy for help, they heard that just 80 homes in their neighborhood had lost power — out of more than 100,000 across Pinellas County.

It became clear, Miller said, that her neighborhood would not be the priority. So she started making readjustments to a time before technology.

“I need my cellphone. It wakes me up in the morning for work. I need my air conditioner at nighttime,” she said. “Can’t cook. Can’t see. Can’t do anything.”

Officials warned that flooding from Florida to South Carolina could pose a particular danger in coming days. Residents around Charleston, S.C., were urged to avoid the city’s downtown until flooding there subsides.

Irma’s thrashing winds cut power to two-thirds of all power company customers in Florida, totaling more than 6.5 million customer accounts. Because each account often represents more than one person, the overall number may be historic, said Eric Silagy, president and chief executive of Florida Power and Light (FPL), the state’s largest utility, which supplies power to about half of Florida. Silagy said Monday that as many as 9 million people were affected by his company’s outages.

Shawna Berger, a spokeswoman for Duke Energy, said 1.2 million of its 1.8 million customers were without power in Florida and noted that if you multiply that number by 2.5 — per the latest census data, she said — that shows that 3 million people were affected.

“We’ve never had that many outages,” Silagy said. “I don’t think any utility in the country has.”

Beach resident Amela Desanto walks along Fort Lauderdale Beach Boulevard, an asphalt roadway covered with sand, to her condominium on Monday. (Andrew Innerarity/For The Washington Post)

The outages pose a particular issue in Florida, where temperatures in Miami and Tampa are forecast to get into the 90s this week. Silagy warned that some people “could be out of power for weeks,” particularly if crews need to rebuild parts of the sprawling electrical system. The utility has sent out 19,500 workers across Florida to restore power and is trying to secure more crews from out of state.

Because of the storm’s size, FPL crews were not able to start restoration efforts until late Sunday night, Silagy said. And they are still not able to move across northern Florida, he said, with debris and flooding impeding their way.

The blackouts extended to surrounding states, with more than 146,000 power outages in South Carolina and outages trending upward in Georgia on Monday night as the remnants of Irma passed through.

As a testament to Florida’s fortune, Caribbean countries preceding it on Irma’s path continued to struggle to recover Monday long after the storm had passed. In Cuba, the hurricane’s scissoring winds and strafing rain had torn apart buildings and roofs and sent flooding along the northern coast. The storm ravaged the Virgin Islands, devastated Barbuda and pummeled other islands on its path.

Irma is expected to keep losing force as it continues inland, and forecasters say it should be a tropical depression by Tuesday afternoon. But the storm maintained its remarkable reach, with tropical-storm-force winds reaching more than 400 miles.

As the storm moved inland Monday, it continued pouring torrential rain onto Georgia, the Carolinas and Alabama, where President Trump declared a state of emergency on Monday night.

In Atlanta, Delta Air Lines canceled about 800 flights from its hub operations Monday in anticipation of “strong crosswinds,” which could reverberate through the air travel system nationwide. Thousands of flights already have been halted due to the storm. Atlanta, hundreds of miles from any coast and more than 600 miles north of the place where Irma first hit the mainland, was placed under its first tropical-storm warning.

As the skies began to clear, hordes of evacuees inland began making plans to return home — a mass migration that had Florida officials pleading for patience and more time.

Nearly 6 million people were told to evacuate ahead of Irma, in what is believed to be the largest evacuation in American history.

Many roads remained blocked by heavy trees, authorities warned. Fuel also was a concern, with some seaports closed and tanker trucks unable to refuel gas stations along the homeward path of many residents.

“Wait for direction from local officials before returning to evacuated areas,” Scott told evacuees in a tweet.

Driving in many cities remained extremely hazardous — an exercise in vigilance due to downed trees and the ubiquitous palm fronds that lurked in wait like alligators on the street. In Miami, some residents expressed frustration about the evacuations, which in many cases ultimately weren’t necessary.

“Everyone got stirred up, and they were told to leave,” said Sara Edelman, 29, a biologist walking along 104th Street with her mother, Philis Edelman, 60, an officer worker. “And now there’s no one to clean the trees up.”

Dan Zumpano, 44, who lives nearby, said he believes authorities began evacuations “way too early” in an abundance of caution, driving people from places that ultimately weren’t seriously impacted by the storm into areas that were: “I thought it was the right thing to do, but I think they sent a lot of people right into the core of the hurricane.”

That was a familiar story: People who evacuated from Miami to Tampa. And then, in some cases, from Tampa to Orlando. The storm followed many of them the entire time. “Every day you saw the models changing,” Zumpano said.

But all along Miami’s streets, signs also remained of the hurricane’s fury and the tragic possibilities that might have been.

Sailboats on Miami’s Coconut Grove marina were flipped over. Million-dollar yachts were half submerged in the bay. Once-idyllic parks looked like desolate war zones. Large trees toppled over, roots dangling in the air.

Resident Paul Plante came to the marina to check on his home and boat, which he had docked indoors. His boat was fine, and he and his sister looked in disbelief at the submerged boats in the bay that weren’t so lucky.

“You have to take nine different roads to get here now, but everything was okay,” he said. “The storm surge could have been so much worse. We’re lucky.”

Chris Perez stands near a downed palm tree that landed on his family's home during Hurricane Irma in Palmetto Bay on Monday. (Matt McClain/The Washington Post)

Zezima, Berman and Wan reported from Washington. Angela Fritz and Sandhya Somashekar in Washington, Darryl Fears in Orlando, Perry Stein in Miami, Patricia Sullivan in Estero, Fla., Lori Rozsa in Gainesville, Dustin Waters in Charleston, S.C., and Scott Unger in Key West, Fla., contributed to this report.