The diving in Cuba isn't bad. You won't find typical vacation divers. There aren't many Americans (some fly to Canada or Mexico then fly to Cuba; US government doesn't allow flights from US to Cuba). Because of embargoes from US, it is hard to get things like soap, shampoo, conditioner, toilet paper, etc. So I usually bring these things unopened. Then I put them in a paper bag with a note saying they are a tip to the hotel staff. Giving $5 of soap is worth a LOT more than $5 cash because it is hard to get good soap. Bring some toiletries for yourself. It is generally inexpensive. I did find they tend to over cook the food. So if you like fish and meat you want to force them to not over cook things.
There is some diving on the north shore. There is a peninsula called Varadero. This is where all the touristy resorts are. The diving here is okay but nothing great.
There is better diving on the south shore. There is a point directly north of Jamaica and west of Guantanamo Bay. Dive shop I worked at likes to go to Club Amigo Marea del Portillo resort. If you google for this you will find Trip Advisor reviews. I think Sunwing Vacations are the only people who fly there. You can fly to Toronto (YYZ) and then catch a Sunwing flight to Cuba from there. Unless there is a flight from Europe direct to Cuba.