A little late, but my daughter and I dove with Native Spirit a couple of years ago, and loved them. We were there in July, and were their only divers the week we were there. Private dives all week!
Lots of good dive sites. Larger fish are south of Point Salines, but those sites tend to be rougher in terms of water conditions. Don't bother with the underwater sculpture garden unless you are willing to research the various statues ahead of time. Without knowing what the artist was trying to convey, they are a bit creepy.
If you are in Grenada, do try and see the rest of the island. The rainforest waterfalls are beautiful. Also try to get to Carriacou for a day. Carriacou is even more laid back than Grenada, and is supposed to have good diving as well.