CHICAGO (MarketWatch) — Already reeling from a real-estate crisis and deep economic slump, Florida faces yet another financial cataclysm if oil from the Gulf spill mars its famous shores, scaring away crucial tourist traffic and wreaking havoc on its fisheries.

Beaches are big business in the Sunshine State. At stake there alone are hundreds of thousands of jobs and perhaps billions of dollars in revenue, depending on when and where the oil from BP PLC’s (BP 39.27, +1.61, +4.28%)runaway well makes landfall.

Although the beaches were still in the clear as of Thursday afternoon, widespread reports of vacation cancellations are already coming in. Also, the pace of new bookings in many areas has slowed, especially in the Panhandle, which looks likely to be the first — and maybe the worst — area affected.

Ocean tourism (as opposed to that offered by Orlando theme parks) and recreation are among Florida’s main industries, contributing an estimated $20 billion a year to the state’s economy, data from the National Ocean Economics Program show. In 2008, 84.2 million visitors spent over $65 billion in Florida, supporting the more than 1 million residents directly employed by the tourism industry, according to Visit Florida, the state’s official tourism-marketing arm.

To stay ahead of the slick and reassure jittery travelers, Florida tourism officials are taking to the airwaves with an ad campaign — funded by a $25 million grant from BP.

“Even though there are no physical impacts to Florida’s shores from the oil spill, the state’s tourism industry, especially in the Panhandle, has already felt a very real economic impact,” said Gov. Charlie Crist, in announcing the first phase of the campaign. “It is vital that travelers throughout the nation and the world know our beaches and waterways are clear and open for business.”

That may not last long. Oil has been sighted 10 miles off the northwestern city of Pensacola, Fla., while more than a third of the Gulf of Mexico has been declared off-limits to fishing by the National Oceanographic and Atmospheric Administration — a swath that includes waters off the Panhandle as well as Key West.

On Thursday, Crist sent a letter to Commerce Secretary Gary Locke requesting the determination of a “commercial fishery failure,” citing “immediate and devastating impacts” from the spill. Such a finding could trigger an infusion of federal aid.

In the letter, Crist blasted news reports for fostering “the mistaken impression that the entire Gulf of Mexico has been tainted” by oil. “This misinformation has affected tourism and seafood consumption in Florida and resulted in severe economic impacts throughout the state.”

On the tourism front, early results, especially over the Memorial Day weekend, were relatively encouraging. But some of that could have been driven by people rushing to get in a last dip before the oil arrives. If pictures of soiled sands and dead wildlife like those coming out of Louisiana, Alabama and Mississippi start emanating from Florida, it could be disastrous.

“It is already ugly,” said John Fareed, a partner at Fareed Zapala Koepke, an Orlando-based hospitality-industry consultancy. “When it hits, it will be real and will position itself in the psyche of consumers who are getting ready to make vacation plans. It is going to have a huge impact in terms of future bookings and cancellations.

“Every indication from the people we work with is that bookings have slowed to a trickle. It is the absolute worst time of year to be dealing with these issues. There is absolutely no good spin you can put on this,” he added.

Fears of lasting damage

Nor does Fareed see this as a short-term problem. “If oil really does start to wash ashore in big waves, people are going to put off coming to parts of this country for a long while.”

The pain will be felt from mom-and-pop motels and restaurants to hotels and resorts operated by global lodging giants, including Marriott International Inc.(MAR 34.37, +0.87, +2.60%), Starwood Hotels and Resorts Worldwide Inc.(HOT 47.04, +1.41, +3.09%), Intercontinental Hotels Group PLC(IHG 16.67, +0.32, +1.96%) and Hyatt Hotels Corp. (H 39.83, +0.67, +1.71%) — some of which have dozens of properties in the state.