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Mr. Obama’s recent two-day visit to Panama City, Fla., where he ate local food and dipped in the water, will help some, Mr. Thompson said.

“The president and the first family - that’s the highest-profile visitors you can have to your state,” he said. “That was a strong statement. A high-profile person like that going swimming and having his family in there, that just reinforces the fact that there’s nothing to worry about.”

Government scientists say 4.1 million barrels of oil spilled into the Gulf since the Deepwater Horizon drilling rig exploded April 20.

Analysts still debate what became of the oil, much of which can’t be accounted for. That uncertainty has even those whose livelihoods depend on the Gulf issuing warnings about safety.

“They said oil’s gone. It’s not gone. It’s on the bottom,” Acy Cooper Jr., vice president of the Louisiana Shrimp Association, told a congressional hearing Thursday. “You stir the bottom up and oil comes up. … Ninety percent is still there and it is going to come into our shores eventually. Somewhere, if not in Louisiana, somewheres else.”

Reports that emerged Thursday would seem to back up his account.

A team of scientists from Woods Hole Oceanographic Institution in Massachusetts mapped an invisible 22-mile-long mist of oil with special instruments that look for the chemical signature of oil.

The plume consists of droplets too small for the eye to see, more than a half-mile down, said researchers who mapped it with high-tech sensors.

Scientists said the most troubling thing is the slow process of degrading, meaning it could be a long-lasting threat for marine life deep in the ocean.

But Mike Voisin, CEO of Motivatit Seafoods, told the same hearing that the seafood being shipped out of the Gulf is undergoing more testing than ever and is safe.

“We have 7,500 miles of shoreline in south Louisiana, if you go in and out every bayou and around every bay and lake. Only about 400 miles of those were oiled,” he said.

Those conflicting messages are unlikely to help the confusion about the amount of damage to the Gulf.

Mr. Hutchinson said the least he can ask of anyone considering a visit to the Gulf is to call the location and ask for advice.

“Before you make a decision not to come, make sure you’re informed. Check with the destination,” he said, adding that they’re looking for repeat customers so they’re likely to be straightforward. “The travel industry and the tourist industry is not about having people come one time and not again.”