I spent a week diving in Dominica this past June. My wife and I had a terrific time. The diving is similar to Bonaire in that you will mostly see small fish and critters, etc.
We stayed at Castle Comfort Inn, which was rather small, but very cozy with good meals, etc. We also stayed for one night at Fort Young Hotel (due to our flight being delayed for one day due to weather), which was more upscale, but not as cozy. We used Dive Dominica (on the grounds of Castle Comfort Inn), which we would recommend without reservation.
Aside from the diving (which to date is my wife's favorite, and my second favorite (second to Bonaire)), Dominica offers a lot of opportunities on land. The hiking is incredible, and if you go, you should definitely take time to hike to the Boiling Lake, Middleham Falls, etc.
All in all, I would say that Dominica has been our favorite vacation spot to date, and we will definitely go back one of these days.
Feel free to pm if you have any further questions.
Jacob