I did Blue Grotto last Friday. My view of Grottos had been tainted by some of the "ugly" ones I had been in. Blue Grotto was different, and was very scenic and clean. I was with a certified Cave Dive Instructor, and we went down to the bottom, at the lowest point, which is about 91 feet right now. Once you get to the bottom it's basically a cave dive, but under controlled circumstances with the permanent guide ropes in place. You circle back around, through the cave, to the other side, where it becomes a cavern dive again. Some might call the whole thing a cavern dive, because even at the lowest point you can see the faintest evidence of light.
There is a rope at around 50 feet marking where to stop if not a cave diver.
It's pretty cool going into the air bell to talk at 30 feet. You should try that. Just stay out of the caves. The only reason I did it was because I was with my instructor, and he knew the grotto very well.