I've been twice and dived with Aquanauts, Scuba Teck, and Dive Grenada, and over to Carriacou with Carriacou Silver Diving for 1 1/2 days. All were good, each rather different. On Grenada most were wrecks or a mix of a reef and wreck. The reefs seemed in reasonably good condition, no big walls more level with a good mix of critters. Carriacou had reefs only, but with steeper terrain.
Aquanauts is the big outfit; a number of larger good boats but slightly more tourist oriented because of this. Dive Grenada, bottom of the Flamboyant, is a smaller operation so depending on the day dives could last longer as more personal. They are on Grand Anse beach and sometimes have cruse ship guests who need to make it back to the ship. Scuba Teck were the most personal (I was even allowed to borrow Rob's rebreather for one day) as both small and a little more isolated to reach but Rob Bryant has sold the business and I don't know the new owners.
As for other things. A car is incredibly useful and simply explore, the north of the Island is different from the more popular south, the mountains different again. The people are very friendly. I certainly enjoyed my stays including in the Flamboyant.
Charles Stirling