Can't give first hand experience, but with no responses yet, I figured i would provide what I could...
We just got back from Grenada on Aug 2. Stayed at the Radisson and dove with Native Spirit. Saw the Sandals dive boat a few times.
From what I understand, Sandals recently got a second boat to be better able to a boat to diving and be better able to do two tank dives. I believe that boat was only expected in late April this year, so you may have to check with Sandals directly unless someone here has visited very recently. I can definitely say that the summer months seemed VERY quiet from a tourism perspective. Our hotel was only at a fraction of capacity, and my daughter and I were usually the only ones on our dive boat. There were only ever a few divers on the Sandals boat despite what looked to be an otherwise very large capacity.
I also understand that some dives with Sandals carry a surcharge. The Bianca C is one of those dives. So is the underwater sculpture park. While the sculpture park was somewhat interesting, I am not sure that I would bother with it again. If they go to the Bianca C, it would be worth the surcharge, as the surcharge would be far less than what you would pay through another operator. It may not be a regular destination though, depending on the mix of divers at the resort at any given time. None of the other operators that I looked at had regularly scheduled dives to any site. Rather, they consider the divers going out and plan sites a day or two in advance based on diver ability, what they have already seen and interest in a particular site.
If you do need to source another operator, I was quite impressed with Native Spirit. Eco-Dive at the Coyaba resort also looked good. Did not see other operations out as the week was VERY slow for everyone. A single tank dive at another operator will run around $60US and a two tank will run around $110-115.
I was actually quite impressed by all of the reefs that we dove. They looked to be very healthy with little evidence of bleaching, etc. If you get the chance to dive some of the sites more to the south with Atlantic exposure (eg Shark Reef, King Mitch), you are more likely to see some much larger life. The more protected Caribbean sites had smaller life, but those sites were also much more protected.
Do take the chance to explore a bit of the island if possible. We had a splendid hike into the Seven Sisters waterfall. It can be muddy after much rain, but our hike was pretty dry. Concord Falls is more developed, and there will always be other people there. Being the only people at a secluded rainforest waterfall like Seven Sisters was far more beautiful.
If you get chance to see the nutmeg and cocoa processing, it is also of interest. You must also try some of the locally grown and processed (tree to bar) chocolate. Delicious!
Don't bother with the River Antoine Rum distillery. While it is interesting to see how rum was made in the 1700s (on some of the same equipment!), the actual product you sample must be what drinking lighter fluid is like. I like rum, but that one is one of the harshest imaginable.
If you do have the chance to explore some of the island, be prepared that there is still a lot of damage visible from Hurricane Ivan still visible. While the reefs and rainforests seem to have recovered well, there are still a lot of abandoned shells of buildings, churches without roofs, etc.
Have a great trip! The island is beautiful, relatively non-touristy, and the people are incredibly warm and friendly.
