Hmm, I would dive every stop on that trip, either because I've been there and know it's good, or I've heard enough to be curious. Maybe I should look up that cruise...
When places have no diving "reputation", sometimes it's because they're not good, and sometimes it's because they're small/not well traveled/inconvenient to get to. I'd say lots of the latter in this case.
I've dove in Carriacou, Grenada, St. Vincent, & Bequia - St Vincent and Bequia land based, and all of them from a liveaboard. All good diving, otherwise a liveaboard wouldn't waste time there.
I'd say the best is St Vincent, I've actually gone there many times, and on the liveaboard the passengers opted to stay there extra time. Grenada was my least favorite, mostly because the viz sucked when we were there, but I don't know how typical our dives there were. Both are pretty scenic and interesting landside.
I've heard very good things about the Tobago Cays, but haven't been there. That would be my first choice since they're probably about the hardest of the lot to get to, and you'll be there!)
I like diving in Bequia, but it's such a neat little island I might opt to just go wander around on land instead, if I didn't want to dive every stop. On the other hand scooting around there in a RIB could be fun too. Try to see Moonhole if only from the water.
St. Lucia has a rep for having some decent diving, it's certainly going to be better known than the little places, but unfortunately I can't compare. I know nothing about Martinique.
Like most places, local knowledge is important to get you to the best sites and spot the local critters, hopefully your ship DMs can do this. It's especially true someplace like St Vincent, which is actually somewhat well known for having lots of unusual and uncommon little critters for the Caribbean, which may be hard to find otherwise. We've had dives with multiple frogfish and seahorses in 6 assorted colors. (Quote from my husband - "I already have a picture of an orange seahorse, can you find me a yellow one?" I think he did, too.) Drawback to diving St Vincent from the cruise, it's a larger island, you will probably be in the south, the diving typically gets better the further north you get, the ride is sometimes "exciting", so I'm not sure how far you will be going there in a RIB. I would probably do it anyway.