“The law is clear: If the trip was allowed, the Helms-Burton does not apply,” Mr. Fowler, who worked on the 1996 legislation, told the paper. “It was not the intention of the Helms-Burton law to go after American companies with legal business in Cuba. They can try it, but I’ve been here for 40 years, and I tell them: Good luck.”

John S. Kavulich, head of the U.S.-Cuba Trade and Economic Council, said the key to the lawsuits will be proving that when the Obama administration expanded the ways Americans would be allowed to travel to Cuba, it broke the law. He said a court could decide that sailing aboard a luxury cruise liner does not qualify for any of the categories permitted under the law, like educational travel.

“If it’s tourism, it wasn’t legal,” Mr. Kavulich said. “If it was legal, then the cruise companies are off the hook.”

Both of the lawsuits against Carnival were filed by Roberto Martínez, a former United States attorney in Miami who has won enormous verdicts against the Cuban government in a variety of cases, including a $188 million wrongful death suit over the four people who were killed when the Brothers to the Rescue planes were downed. Frozen Cuban assets in the United States were used to pay some of those awards.

Mr. Martínez said his clients had been preparing for years and were more than ready to file suit on the first day they could.

“Their family businesses were destroyed, stolen by the Castro government, and these American companies were put on notice for many years that they were using properties that were stolen, and they did nothing about it,” he said. “They miscalculated the decision that it was worth doing business and ignoring the pleas not to use stolen properties, and now they are basically going to court and having to deal with the consequences of that risk.”

He said the law stipulates that former property owners can seek triple the value of the property as compensation, and the property can be valued several different ways. The Havana Dock Company has a claim certified by a United States commission saying that its property was worth $9.1 million in 1960; under the law, it could be awarded three times that amount, plus interest, or three times the current fair market value.