All of the islands that you have cited are suitable for newbie diving provided that one does not seek out the most challenging sites, which obviously is not your plan.
As regards quality of newbie diving, Barbados, St. Lucia & St. Thomas are probably best. If you're not intending to dive at every port, I'd suggest passing on scuba at St. Maarten & Antigua.
As you have alluded to, there are pros & cons to diving with the ship's op versus your own selection, and time considerations are primary among them.
I'd suggest that you begin by reading this edited piece on cruise ship diving from my July '00 "Dive Workshop" column in "Rodales' Scuba Diving:"
"Cruise ships are seen by many divers as despoilers of the both the environment & culture at their ports of call. The situation in Cozumel in particular is regularly lamented on several scuba bulletin boards. Nevertheless, cruising is among the fastest growing segments of the travel industry, and bigger & fancier ships are regularly launched, and new piers constructed.
These floating megaresorts generally offer scuba diving as one of many activities, but rarely is it a focal point. This places them the most removed of water borne diving, with dedicated dive live-aboards being at the top. Between these two are the smaller boats operated by the likes of the Windjammer & Law fleets, which cater to those wanting some serious diving, but also a relaxed atmosphere and other diversions. Cruise ships do afford some advantages, however, such as allowing: (1) groups with nondiving members to vacation together; (2) the diver to screen a number of locations cost-effectively and with relative ease; (3) divers who are not into a trip of hard core diving to get an N2 fix; & (4) a convenient venue for a resort course or OW certification--anything beyond this will be conduced by the shore ops.
If you decide to cruise & dive, you can: (1) go with a traditional ship and let them make the arrangements; (2) do the same but arrange for your own diving; or, (3) select a ship specifically oriented toward diving. If diving with an op arranged by the ship, it may be selected based on it's ability to adhere to the ship's schedule, the amount it kicks back, and other factors not related to getting the best diving. If good diving is a priority, you often will be best served by making your own arrangements prior to the cruise. This can require some effort due to the ship's need to keep a tight schedule. It may dock after most dive boats have gone out in the AM, and may depart before others return in the PM. In addition, cruise ship piers are often some distance from the areas where the best diving is located. Definitely make advanced booking if you decide to select your own op.
Princess, Celebrity and Windstar are among your best choices in dive oriented lines. On board dive centers have instructors, and offer at least a resort course, with other certs handled by the on shore op. They endeavor to get you to the better sites with the better shops at any particular port of call, although for very experienced divers they often are not the most advanced sites or personalized ops. Dive gear is typically available on board, and, if not, can be obtained from the local shop. Some ships include gear in the price of the dives, typically $80-90 for a two tank trip, but some charge for rental. Best to check beforehand if looking to save a few bucks, but remember that schlepping your gear from room to shore op everyday, and storing/drying it your cabin, can be a hassle. Also, some ships have odd rules, such as wanting a logbook as well as c-card, or requiring a refresher a course if you haven't dived a while.
Bottom line is, cruise ships are not dedicated dive venues, and will not satisfy those who want nothing but dive, dive, dive or advanced diving. They do, however, provide an opportunity to get in a few dives and several locations while enjoying the other amenities of this type of vacation. DocVikingo"
Do have fun.
DocVikingo
