We stayed a few days and dove on Grenada. We stayed at Spice Island Inn. Quite upscale, pricey and a bit formal....no ahletic shoes in the dining room, etc. We dove with Aquanauts who have a satellite dive shop at the resort. Boats a bit crowded during our stay and frequently dropped you off at their shop on the other side of the island, resulting in a van transfer and lengthy delays getting back to the hotel. Again diving was OK during our visit but not spectacular. The Bianca C wreck was impressive. This was in November of '06.
To be honest the best diving on that trip was at Blue Waters Inn on Tobago. I know a lot depends on conditions during your visit....some luck there. But the coral, fishlife, viz were very good during our visit. Certainly dramatically better than Carriacou or Grenada. Dove with Aqua Marine Diving which operates right at the Resort. Just an average dive operation with modest older boats but they are just steps from your room and boat rides are very short to good dive sites. Good shade on boats. Nothing fancy about Blue Waters Inn. Definitely chose the more expensive rooms. The cheap ones are old and funky. They have a fun bar and serve inexpensive good food (hamburgers, sandwiches, bar chow). We recall the meal plan being a little expensive. Several restaurants in Speyside. Be warned, however, it is a long taxi ride from the airport to Speyside (1.5 hours) but scenic. Also Topago customs/immigration can be incredibly slow. I recall for that trip we flew to Barbados and did short flights from there to Grenada, Tobago, etc.
That's probably more than you wanted to know.