The Coast Guard had been looking off the coast of Pensacola, Florida, for a "missing jet skier" whose wife reported him missing Monday night. Williams said that rescue crews would "temporarily suspend their search efforts until the storm passes and weather conditions become safe enough."
Isaac's impact has been felt, and seen, in other ways as well -- including bending trees, drenching roads and knocking down power lines.
Entergy Louisiana, one of many utilities affected by the storm, reported around 222,000 customers without power statewide at 10:12 p.m. CT.
Isaac earlier prompted three airports to close -- in New Orleans; Gulfport-Biloxi, Mississippi; and Mobile, Alabama -- and cancellations of around 1,500 flights systemwide, according to airline and airport officials. In addition, the U.S. Coast Guard announced it has closed major ports along the Gulf Coast and the Mississippi River from Baton Rouge to its mouth because of the storm.
Business in many of the most affected areas came to a standstill because of Isaac. For example, 52 Walmart and Sam's Club stores in Louisiana and nine in Mississippi had shut down by 5:30 p.m. Tuesday, their parent company said.
Amtrak suspended its train service to and from New Orleans on Tuesday and Wednesday because of Isaac. And residents of Plaquemines Parish, located along the Gulf, are subject to a "dusk-to-dawn curfew" because of Isaac, parish officials said.
In and around New Orleans, authorities closed a number of flood protection structures. These included the West Closure Complex -- one of the world's largest pump stations, capable of pumping stormwater at 20,000 cubic feet per second -- said the Army Corps of Engineers.
"There is no evidence of any (water) overtopping (canals)," Landrieu said Tuesday evening. "We have full confidence the levees will hold."
In Mississippi, more than 1,800 evacuees were staying in 33 shelters located in 16 counties as of 9 p.m. Tuesday, according to the state's emergency management agency. More than 80,000 sandbags were given out earlier in the day, and several popular Biloxi casinos closed temporarily.
Shrimpers in Bayou Le Batre, Alabama, were among those who heeded official warnings and hunkered down.
"All the boats are coming. We're anchoring them down and getting ready for this blow, hoping it's not too bad," Dominick Ficarino, the owner of Dominick's Seafood, told CNN affiliate WPMI-TV in Bayou Le Batre.
The Nuclear Regulatory Commission on Tuesday said it was sending additional inspectors to two Louisiana nuclear plants in the storm's path, as power company Entergy planned a "controlled shutdown" of one of them starting Tuesday afternoon.
Isaac is not expected to be as strong as Katrina, which made landfall as a Category 3 storm with 125 mph winds. Even so, the head of FEMA stressed Tuesday that wind speed isn't the only factor to consider when assessing how much damage a storm might cause.
"I know there are a lot of jaded folks on the coast that have been through Camille and then Katrina and think, well, this is just a Category 1 hurricane," said FEMA director Craig Fugate from Mississippi. "The tendency to look at the category of the wind speed doesn't always tell the story of what the impacts will be."
Gulf Coast authorities and residents are praying there will be no repeat of the devastation the 2005 hurricane caused after protective levees around New Orleans failed and flooded the city.
Col. Ed Fleming of the Army Corps of Engineers vowed that the system now in place "is the best ... the greater New Orleans area has ever seen."
Jackie Grosch, for one, trusts that the levees will hold this time. The St. Bernard Parish, Louisiana, resident, who rebuilt her home after Katrina, said her family will wait Isaac out -- though they have a generator and life jackets, "just in case."
"I don't know if it's going to be a true test, because they're saying it's not going to be that bad," Grosch said. "But you never know what bad is. We didn't think Katrina was going to be bad either."