Yes, traveling to Cuba helps the Castro regime directly and indirectly. The money spent bolsters the flailing Cuban economy, which helps shore up the current power structure. Also, the mere act of visiting normalizes a social and political acceptance of the communist regime. It's like a store; the more visitors to the store, the more business and the more it prospers.
United States citizens that travel to Cuba will in affect be supporting Castro's regime. Yes, the idea of traveling to Cuba is very appealing to many Americans. However, American tourists will mean money for the Cuban government - a government that is still brutal and does not respect human rights. Therefore, Americans wanting to visit the island country should think twice before spending their money down there.
At initial consideration, it is easy to think U.S. citizens traveling to Cuba will merely support Castro's regime. The economy is largely government owned with very little private sector businesses. This has changed in the past couple decades. In the early '80s the private sector was minuscule. With influx of U.S. citizens traveling to Cuba, the economy can be reignited giving Cuban citizens that could only work for the state and opportunity to explore greater work opportunities. Such changes in financial opportunities, may lead citizens of Cuba to pursue changes in the government at a pace the country can handle.
People who travel to Cuba will simply be traveling to Cuba because they want to see it. The vast majority of tourists have no political agenda at all. Phrases like "supporting Castro's regime" sound like government propaganda. In reality, tourists will be supporting local businesses and restaurants owned by people of all political persuasions.