I was looking at a couple of all-inclusives in Dominican Republic so I think it would be nice to relax at the beach or pool too.
The one day I hit Dominican Republic was on a cruise, arriving in Samana, and I did not do a dive excursion because time on the forum suggested the good diving wasn't nearby.
So be aware that where you stay in the D.R. could play a role in your experience. I suggest doing a forum search & reading some trip reports & discussions on Dominican Republic diving.
I did 4 dives in St. Thomas; had fun (2 were training dives; the other 2 were at Armando's Paradise and Navy Barges, if you want a couple of sites to research online). My dives were easy, not too deep, nice Caribbean diving.
But having done 20 dives out of Key Largo last September, I'm not sure why I'd fly to St. Thomas, when I could fly to Florida (I did Ft. Lauderdale; I understand Miami and Key West are options), rent an SUV, drive down & stay at the Courtyard Marriot and dive with Rainbow Reef Dive Center at the end of their parking lot. 2 2-tank trips per day let me cram 20 dives into 5 days. Sweet! Not every dive operator lets you do that.
Different strokes, different folks, but I get the sense you're trying to keep costs down, and as dive trips go, mine was pretty cheap. My 20 dives were a 10 trip $600 package (add taxes and tips, but still, at $30/dive with a free guide provided, can you beat that?).
As for Cancun, haven't dove it. Did 2 dives in Cozumel. Other posters on the forum have indicated Cozumel has superior diving, but Cancun has more for non-divers. I'm about the diving, so haven't explored that option.
Richard.
---------- Post added February 6th, 2014 at 09:11 PM ----------
P.S.: Key Largo/Florida Keys diving conditions will vary more with season than some Caribbean destinations.