Major offshore oil drillers were returning staff to their platforms in the Gulf of Mexico, although operations were expected to take several days to ramp up.
The first hurricane to hit the United States this year will be remembered for striking New Orleans on the anniversary of 2005's deadly Hurricane Katrina - and providing a first, successful test of the city's new $14.5 billion flood controls imposed after Katrina.
"We are now fully in the cleanup and recovery phase of this storm," said New Orleans Mayor Mitch Landrieu.
Isaac left some homes in the state under 12 feet (3.6 meters) of water. More than 500,000 homes and businesses across Louisiana, Mississippi and Arkansas were still without electricity Friday morning.
At least four deaths were attributed to Isaac in the United States and at least 23 people in Haiti and the Dominican Republic earlier.
The storm caused anywhere from $700 million to $2 billion in insured onshore losses, disaster modeler AIR Worldwide said late Thursday.
That would leave Isaac, which came onshore as a Category 1 hurricane, well outside the 10 most costly U.S. hurricanes.
The National Hurricane Center said Isaac, downgraded to a tropical depression, still was likely to trigger tornadoes in the central U.S. Midwest states - among the final acts of a storm that often confounded forecasters and punched above its weight in terms of damage.
Isaac's rain was a godsend for Midwest farmers suffering from the worst drought in more than 50 years. Even if too late for many of this season's crops, the rain will replenish soil moisture in time for winter wheat planting and boost critically low river levels.
Isaac caused widespread flooding and property damage in the U.S. Gulf Coast region, mostly because of its heavy and persistent rainfall. The system lingered near New Orleans for the better part of two days, sometimes moving as slowly as 5 miles per hour (8 km per hour).
Through it all, New Orleans sustained mostly cosmetic damage such as downed trees and street lights.
Life was beginning to returning to normal in the city on Friday, although most of it was still without power after what utility Entergy Corp described as the fourth-largest storm it had ever faced.
National Guard troops had opened three sites around New Orleans to distribute water, ice and military-style prepackaged meals to residents on a warm, steamy day. Gasoline still was hard to find as well.
New Orleans International Airport reopened early on Friday, and the Port of New Orleans also reopened, in time for the arrival of the 2,052-passenger Carnival Elation cruise ship.
Downtown and in the French Quarter, businesses opened, either with generators or without electricity. Most stores had removed boards from windows, and some commuters headed to work.
DOWNED LIMBS, UPROOTED TREES
In residential areas outside the city center, streets were littered with downed limbs and some trees were uprooted. Residents were out clearing debris.
"I am surprised how much debris there is everywhere," said David Doucet, 55, a member of the Grammy award-winning Cajun band Beau Soleil, as he walked his dog in downtown New Orleans. "The trees have had seven years to grow since Katrina but they didn't grow all that strong."
Republican presidential nominee Mitt Romney, fresh from his party's convention in Tampa, Florida, was to visit Louisiana on Friday to view storm damage and meet with Governor Bobby Jindal, who praised citizens' resilience and vowed that the state would emerge "better and stronger" from Isaac.
President Barack Obama, who declared a disaster in Mississippi and Louisiana on Wednesday, is scheduled to visit the region on Monday.
Louisiana Senator Mary Landrieu made a plea for additional federal funds to build protective levees in the state, while noting that the Army Corps of Engineers has a meagre budget for construction projects.
New Orleans' Audubon Park recorded 18.7 inches (47.5 cm) of rain in a 24-hour period during Isaac. That exceeded records dating to 1871, said Jeff Masters of Weather Underground. Many other locations in Louisiana and Mississippi logged more than 10 inches (25.4 cm) of rain.
Surrounding areas, though, without the new protective federal flood barriers, did not fare as well from the relentless rain and huge storm surges.
Some of the worst flooding was in Plaquemines Parish, southwest of New Orleans, where at least one levee was topped, leaving many homes under about 12 feet (3.6 meters) of water.
The state advised residents of flooded neighbourhoods to boil tap water before drinking it.
Slidell, northeast of New Orleans, took the brunt of a storm surge from Lake Pontchartrain, which left some neighbourhoods under about a foot (30 cm) of water - much of which had receded by Friday.
"You'd have never made me believe a Category 1 would dump this much water," said Sam Caruso, 71, a former mayor of Slidell who toured the town in his pickup truck on Thursday.
As the flood waters rose, some residents, including Caruso, wondered whether the new federal levee system had shored up New Orleans at the expense of low-lying neighbouring parishes outside the system's protection - a debate that is likely to continue.
Brent crude oil was up $1.90 to $114.55 a barrel on Friday, although major oil facilities on the Gulf of Mexico made it through Isaac mostly unscathed.
BP Plc and Royal Dutch Shell both said they were returning staff to their Gulf of Mexico offshore oil platforms. Production could take several days to ramp up to pre-Isaac levels.
Louisiana's coastal oil refineries also began to power back up. Most came through Isaac unscathed.
The Department of Energy will loan 1 million barrels of crude oil from emergency reserves to Marathon Petroleum Corp after the firm's Garyville, Louisiana, refinery cut its run rate ahead of the hurricane. [ID:nL2E8JV94R] A larger tapping of the Strategic Petroleum Reserve is possible.
Storm watchers have turned to Tropical Storm Leslie, currently 845 miles (1,355 km) east of the Leeward Islands with wind speeds picking up to 65 miles per hour (100 kmph). Leslie could become a hurricane over the weekend, posing a potential threat to Bermuda next week.
(Additional reporting by Chris Baltimore and Kristen Hays in Houston, Sam Nelson in Chicago, Ben Berkowitz in Boston and Lisa Lambert aboard Air Force One; Writing by Ros Krasny; Editing by David Adams and Bill Trott)