Hi,
My wife/dive-buddy and I just did a Royal Caribbean cruise heavy with diving in March. A lot of the advice and opinion presented already is good, but as always there is a lot of personal desire and choice and taste involved, so I will simply present my take on things for whatever information or insight (or amusement ;-) that might provide:
First, we like cruising for vacations a lot (diamond members now on RCCL). We have done a lot of the excursions on repeat ports already (typically the active stuff like snorkelling or kayaking), so scuba is a new way to enjoy the ports we revisit. In fact, we got enticed to diving from our experiences cruising (love snorkeling, watching divers below us thought "this is cool, but that's really COOL").
Second, we are relatively new divers. We love it, and have done some dedicated (short) diving trips (to Cozumel), and local diving (in Chicagoland, brrrrr, but want to build the skils and dive). But, we've only been at this a year or so, ~45 dives.
Third, the excursions:
The diving excursions done from cruises are aimed at beginning to next step more experienced divers. You can still have good diving if you fit that profile.
The excursion operators are reasonably audited and held to high standards of operation by the cruise line (reducing the need for doing your own research in a new area). You can have confidence in a good operation (full safety equipment, roll calls, dive briefings, etc.). The dive ops and cruise line collaborate on scheduling to fit your port time, and guarantee your return to the ship (something less certain when booking your own). This is what you are paying the premium for.
You do get a nice sampling of a bunch of diving locations; but it is a quick fly-by sample. Cozumel was great, as was St Thomas, St Maarten, and Nassau. I don't think we got to give Belize a good chance, though, it is too spread out to get a good sense of the place in one short day.
So, cruise diving seemed a good thing, and we are at a sweet-spot in terms of timing ... still wanted the cruise vacation, wanted to dive more and our experience level matched the excursion target market.
The ship we took, Exlporer of the Seas, has a Sea Trek dive shop onboard (they manage the excursions), which turned out to be useful: Our dive computers failed on the first dive (fortunately we had backup depth/bottomtimers, and always plan against tables also), and we could rent (and later buy, it was a good deal) replacement computers. I think having an onboard dive shop is a good feature to look for.
The downsides noted are also quite real. It is a pain to lug all your own equipment, hard to pack for all the disparate things you do (diving, formal dinners, etc.), and you definitely benefit from the balcony and tub suite ($$, though many cruisers will do this anyway if they can afford it). We brought everything, since we were diving a lot this trip, but we did observe the quality of rental gear was good (you could rent at the onboard dive shop, or at the excursion operator), so keep that option in mind for some of the gear if you don't want to lug.
The dives were indeed targeted at lower experience levels, but we were pleased to see that most of the ops were able to split the boat into different groups and tailor the experience more appropriately to the variations amongst the divers. Still, very experienced divers would probably not find the dives great (there was one such pair ... thought it was nice to dive at all while cruising, but weren't thrilled).
It was certainly a nice way to combine two things we live to do, and the timing where we can successfully do so was perfect. In not too long, I expect we will do more diving-only vacations, and find cruise diving at best a nice diversion while doing a cruise anyway.
Bottom line: For us, with our vacation desires and diving experience, cruise diving was terrific. However, do recongnize that the experience can be a specialized fit!
Cheers,
Walter
