Irma slammed into southwest Florida on Sunday, threatening to bring dangerous storm surges to coastal areas already battered by the hurricane's powerful winds.
Now a Category 2 storm with maximum sustained winds of 105 mph, the National Hurricane Center said in its latest advisory at 8 p.m. ET.
Irma is already uprooting trees and turning streets into rivers.
Forecasters warn that water levels are rapidly rising in Naples as the storm churns north.
And there's plenty more to come as Irma climbs the coast toward Fort Myers and Tampa.
"We're all hanging in there, ready to get out there to help others as soon as it's safe to do so," Marco Island Police Chief Al Schettino said as the storm hit his city on Sunday afternoon.
Even areas that aren't facing a direct hit from Irma are seeing flooding and downed power lines.
"We're getting slammed right now," Josh Levy, the mayor of Hollywood, Florida, said on Sunday afternoon.
Expected to be even more dangerous than the powerful winds are the storm surges that threaten Florida's coastal cities.
"The threat of catastrophic storm surge flooding is highest along the southwest coast of Florida, where 10 to 15 feet of inundation above ground level is expected," the hurricane center said. "This is a life-threatening situation."
Still, not everyone heeded orders to evacuate coastal Florida.
Wayne Ploghoft hunkered down Sunday on the third floor of a building on Marco Island -- where life-threatening storm surges are imminent.
Ploghoft said he wasn't able to evacuate because his flight plans didn't work out. Now Ploghoft and three others are holed up with stockpiles of water, canned food and battery power.
"We're all going to be OK," Ploghoft said.
Gov. Rick Scott said Irma's wrath is unprecedented.
"We have never had anything like this before," he said Sunday.
In Florida and southern Georgia, more than 8 million people face hurricane-force winds topping 74 mph, said Ryan Maue of WeatherBell Analytics.
And almost the entire state of Florida is under a hurricane warning, affecting at least 36 million people.
Miami faces Irma's wrath
Gusts topping 90 mph whipped Miami on Sunday, knocking out power to more than 750,000 customers in the Miami-Dade area.
Flying objects such as coconuts turned into dangerous projectiles. And at least two construction cranes partially collapsed. One swung vigorously over downtown Miami. Another dangled over the city's Edgewater neighborhood.
Matthew Spuler captured video of waves crashing over a seawall toward his downtown high-rise building.
"There is no seawall whatsoever," Spuler said. "It's amazing. It's under water."
The latest developments:
-- Water levels are rapidly rising in Naples, Florida, the National Hurricane Center said Sunday evening. A tide gauge in Naples has measured a water level of 2.2 feet above mean higher high water, a 7 foot increase over the past hour and a half, the center said in an advisory at 6 p.m. ET.
-- More than 2 million electric customers are without power across Florida, according to utility companies.
-- A storm surge warning wraps around the state, from Brevard County to Tampa Bay.
-- Miami-Dade County announced a curfew between 7 p.m. Sunday and 7 a.m. Monday. A 6 p.m. curfew has been put in place for Tampa. Manatee County officials announced a curfew from 3 p.m. ET Sunday until 3 p.m. ET Monday.
-- Georgia Gov. Nathan Deal has expanded a state of emergency to include all 159 Georgia counties, Deal's office said. The decision was made in advance of heavy rains, strong winds and potential flooding from Hurricane Irma. The state government will be closed Monday and Tuesday except for essential personnel.
-- At least 24 deaths have been blamed on Irma in the Caribbean islands, where it hit before barreling toward Florida.
'You can't survive these storm surges'
Florida's governor warned that storm surges could be deadly.
"You can't survive these storm surges," he said.
Those who did not evacuate ahead of the storm are in danger, Federal Emergency Management Agency Administrator Brock Long said Saturday.
"You're on your own until we can actually get in there and it's safe," he told CNN.
But Key West business owner Jason Jonas said he stayed behind this weekend because he's in a home that is "built like a bunker."
"It's pretty much the only reason I considered staying here because I knew that I had a pretty good chance of making it through this thing," he said.
"We're 30 plus feet above sea level and in a place that's built to withstand 225 mph winds -- I mean that's a better chance than being exposed out on the highway in traffic trying to make it to Georgia."
Irma hit Cuba's Ciego de Avila province late Friday as a Category 5 hurricane before it weakened and headed to the United States.
This is the first year on record that the continental United States has had two Category 4 hurricane landfalls in the same year.
Last month, Hurricane Harvey devastated much of coastal Texas and killed more than 70 people.
Other cities will get pummeled
Several Florida cities are in or near the forecast path of the storm's eye.
The storm will be devastating for central Florida, Tampa, Fort Myers and Key West, CNN meteorologist Chad Myers said.
In Fort Myers, where storm surge warnings are in effect, Evanson Ngai stayed up all night Saturday, tracking the hurricane.
"I've tried to get some sleep but I can't. Just the nervousness, trying to keep an eye on it to see if its track will change," he said.
Ngai said he planned to crouch in the bathtub when the storm makes landfall.
"Right now, it's a little bit of gusty winds and some rain," he said early Sunday. "We've moved everything away from windows. We're hoping for the best -- we've bought nonperishable foods and water, and we have a flashlight."
Florida Power and Light estimated 3.4 million of its customers could be without power at some point during Irma, the company's largest number of outages ever.
"We think this could be the most challenging restoration in the history of the US," company spokesman Chris McGrath said.
Other states may be affected
As Irma moves inland, more than 45 million people will face tropical storm conditions -- meaning winds will top 39 mph, Maue said. Affected states include Florida, Georgia, Alabama, Tennessee, South Carolina and North Carolina.
South Carolina Gov. Henry McMaster issued a mandatory evacuation for some barrier islands.
The National Weather Service in Atlanta issued a tropical storm watch for the area Monday and Tuesday. Schools in the state planned to close Monday.