During one simulation, nearly a dozen government vessels maneuvered off Fort Lauderdale to stop a craft supposedly carrying armed smugglers headed to Cuba to pick up migrants.
That simulation began hours after a real Border Patrol mission picked up more than 40 Spanish-speaking migrants who happened to arrive along Miami-Dade beaches. Arrivals like those occur often in South Florida and involve mostly people from Cuba.
But Cuba experts said they don't expect massive waves of migrants reminiscent of the 1980 Mariel boatlift - even after the death of Castro, who transferred power to his brother Raul last July because of ill health.
More than 124,000 people were stopped at sea in a six-month period during the Mariel crisis, which was triggered when Castro said anyone who wanted off the communist island could leave.
"Forget it. It ain't gonna happen," said Jaime Suchliki, a University of Miami professor and the author of "Cuba from Columbus to Castro."
"Raul would have to say, â€˜Anyone who wants to go, go,'" Suchliki said. Such a move would destabilize the Cuban government and cause another major crisis with the United States. Raul Castro wants neither, he said.
Also, he said, the region doesn't have enough vessels to transport half a million people out of Cuba. A more likely scenario would be thousands attempting to get into the U.S. naval base at Guantanamo Bay, Cuba. "What would the U.S. do in that scenario?" Suchliki asked.
Cuban activist Ramon Saul Sanchez, head of the anti-Castro group Democracy Movement, said the focus on mass migration was overblown.
"If there is a total breakdown of the Cuban government, people will see the possibility of a democracy and freedom much closer - in their own country. They are coming here because of the tyranny," Sanchez said.
Sanchez, who took to the seas to rescue Cuban rafters during a smaller migration crisis in 1994, was concerned that the U.S. government might stop exiles from attempting to bring humanitarian aid to the island during a change in government.
Border Patrol spokesman Steve McDonald agreed there was no immediate threat of mass migration but stressed the need to train anyway. "We as public officials have an obligation to be prepared," he said.
Reporters were permitted to ride along for Thursday's exercise, which began with a mock 911 call to the sheriff's office reporting that armed boaters were headed to Cuba.
By midmorning, two helicopters and nearly a dozen vessels from the Coast Guard, Customs and Border Protection, and local law enforcement trailed the craft supposedly carrying smugglers. The pursuing boats flashed their lights and ordered the vessel to stop.
Agents did not attempt to board the smuggling boat because of choppy seas, nor did they attempt to snare or disable it out of concern that doing so would risk giving away law enforcement tactics.
With 85 agencies participating, the exercise was the largest since a 2003 presidential directive created the Homeland Security Task Force Southeast to better police the nation's southeastern borders.