Dive Grenada has had some positive reviews on Scubaboard:
Grenada diving?
Grenada Report
I dove with Native Spirit in 2013 and had good experiences with them. We were there in August though, so it was very quiet and we effectively had privately guided dives each day since we were the only divers with them that week.
Coyaba is definitely one of the nicer resorts along Grand Anse. We have not stayed there (have stayed at the Radisson and Blue Horizons resorts, and made extensive use of facilities at Spice Island as well), but have walked the grounds and were impressed with what we have seen on our visits to Grenada. I would definitely consider staying at Coyaba, and would also revisit any of the resorts that we have stayed at in the past.
We enjoyed pretty much all of our diving. If you get the chance, try and get "around the point" of the airport to some of the southern sites. This is where the larger pelagics will be. Mostly smaller fish off Grand Anse.
The sculpture garden is a very common site, but was underwhelming. If you have to dive it, I strongly recommend reading a bit about the sculptures first (
Underwater Sculpture Park | Scuba Diving | Explore). Without at least a bit of understanding of what you are looking at, it gets creepy fairly quickly.
If you go, do take the opportunity to explore the island a bit. The rainforests and waterfalls are stunning. If you get the chance to visit Carriacou, please do. I did not dive there, but understand the diving there is fantastic. The island itself is delightful and so laid back makes the rest of the Caribbean seem somewhat "hustle and bustle"!
Finally a couple of bits of advice. First-- avoid going into St Georges if there are cruise ships in port. The town quickly becomes a zoo. You should be able to see if are any from the resort. If you see any-- go diving instead of exploring. Finally, the island still shows considerable damage from Hurricane Ivan in 2004. On our first visit in 1996, the Carenage in St Georges was a delightful and bustling area. Today (due partly to the moving of the cruise ship terminal, but also the hurricane), it is gritty and industrial.