In my five dive trips to Cuba over 24 years, I have always seen plenty of Americans. Of course, citizens from every other country visited, but Americans had to fly through Mexico, Bahamas, or Canada, use no credit cards, request no passport stamp, and perhaps toss out a white lie or two when returning home. Many divers did, and rarely did one face any issues.
While Barack Obama moved to open relationships with Cuba, last summer President Trump reversed policy on American trade and travel. While his restrictions have caused confusion for American tourists (but not tourists from other countries, who travel there freely), they seem to have had a negligible practical effect.
The U.S. State Department allows organized tours and individuals to travel to Cuba, as long as the purpose falls under one of twelve categories. If you're traveling as an individual, when you make your airline reservation, you will be asked on the airline website to click the menu for your reason for travel: your reason is cultural exchange.
It is illegal to stay in hotels or do business with companies that are connected to the Cuban government (there is a list of restricted business at www.travel.state.gov, which you should visit for the latest information on Cuba travel), but there are plenty of Airbnb rooms.
Avalon will provide a Letter of Authorization (General License), as well. I printed out a copy and kept it with my passport in case I was asked. I wasn't, and my stop at the U.S. customs counter in Fort Lauderdale lasted 10 seconds.
For my January trip, I flew both ways on Jet Blue, but my friends flew on Southwest, United, Delta and American from various U.S. cities -- with cheap fares or modest doses of frequent flyer miles.
Like most divers, I wanted to spend a few days in the capital, so I flew into Havana, an easy, cheap and interesting city. I spent an evening walking around the old city -- food and music were everywhere -- and went to the National Theater for a jazz concert. You can stay in homes via Airbnb or informally, for about $35 per night per room, with $5 pp extra for a nice breakfast.