I am a Canadian and I've been to Cuba three times in the past two years. Each time I have arrived at a different airport - Varadero, Cienfuegos and Holguin. My passport has never been stamped - there is always a separate piece of paper you get on arrival to keep and hand in on departure.
I have got really good last-minute all-inclusive deals from selloffvacations.com or one of the other tour operators advertising in the Toronto newspapers. The package originates from Toronto and it seems like there are a few Americans on board each flight. There doesn't appear to be any problem for Americans getting into Cuba.
The diving on the south coast seems to be a little better - this is the leeward side of the island. I remember doing a great dive close to the Rancho Luna resort near Cienfuegos, descending in a canyon between two huge coral reefs down to three small sunken military craft lying upright in the sand at 161 ft. deep with a water temp. of 82F and vis of about 100 ft. - just awesome. But don't expect to see many large critters - I think they have been mostly fished out for food. I have not been able to get any tanks for shore diving from any resort I've been to - they only want you to do boat diving - I'm sure for security reasons.
The cost for a diving holiday is very reasonable right now, but I suspect that, later, when relations are normalized, as Americans pour in with investment money for new hotels, etc., costs will rise dramatically.