BurBunny
Contributor
If you're wanting the best traditional cruise and dive experience, I'd recommend first a Southern Caribbean itinerary out of San Juan then a Western Caribbean second. The Southern itinerary will often give you 4-5 dive days on a 7-night itinerary, while the Western will give you max 4, but often 4 excellent stops.
Southern islands include combos of some of the following - St. Thomas, Aruba, Curacao, Bonaire, Dominica, St. Lucia, Barbados for the better diving islands (definitely not in that order), and some not high on my list islands for diving of Antigua, Martinique (good, but not logistically from the ship), Trinidad, Isla Margarita, Grenada (haven't yet found an operator there who will work with the cruise itineraries, unfortunately), St. Maarten.
In the winter months, lots of different options for style of cruise by way of cruise line and specific ships, which is just as important as itinerary. August/September quite bit more limited as most of the ships in this area pull out to Alaska or Europe for the summer. But there are a few year-rounds to choose from.
On the western side, several lines have the Roatan, Cozumel, Belize and Grand Cayman itinerary.
For most ports, recommend booking independently if you want a bit more personal experience. In most ports, it's not too difficult or risky with regard to logistics. Work with someone who knows the cruise lines and dive operators
By the way, no such thing as a Bahamas-only 7-night cruise. On a 7-night, you'll have maximum of 1 stop there, in Nassau that time of year. I'd rank other ports as better options. Same thing with Key West. Can be fun ports, but the diving isn't the best the Caribbean has to offer - at least not diving from the ships. There are better cruise dive ports around.
Southern islands include combos of some of the following - St. Thomas, Aruba, Curacao, Bonaire, Dominica, St. Lucia, Barbados for the better diving islands (definitely not in that order), and some not high on my list islands for diving of Antigua, Martinique (good, but not logistically from the ship), Trinidad, Isla Margarita, Grenada (haven't yet found an operator there who will work with the cruise itineraries, unfortunately), St. Maarten.
In the winter months, lots of different options for style of cruise by way of cruise line and specific ships, which is just as important as itinerary. August/September quite bit more limited as most of the ships in this area pull out to Alaska or Europe for the summer. But there are a few year-rounds to choose from.
On the western side, several lines have the Roatan, Cozumel, Belize and Grand Cayman itinerary.
For most ports, recommend booking independently if you want a bit more personal experience. In most ports, it's not too difficult or risky with regard to logistics. Work with someone who knows the cruise lines and dive operators

By the way, no such thing as a Bahamas-only 7-night cruise. On a 7-night, you'll have maximum of 1 stop there, in Nassau that time of year. I'd rank other ports as better options. Same thing with Key West. Can be fun ports, but the diving isn't the best the Caribbean has to offer - at least not diving from the ships. There are better cruise dive ports around.