We just got back from the Bahamas, and snorkled at Stuart Cove's on Paradise Island. It was great. They did 3 stops, the first two were over reefs, and the last was snorkling with sharks. The sharks were reef sharks and nurse sharks, so it was safe, but if you are going with kids, they love it. Admittedly, I have not snorkled very often, but I loved it.
If you have a chance to check out Atlantis, do it. It was unbelievable. They have a huge (I was told it was the largest open air aquarium in the world-12,000,000 gallons) and it was incredible. They only had one small reef aquarium, but tons of tanks, pools and lagoons full of fish, sharks, etc.
Paradise Island is right next to Nassau, and is about a half hour cab ride.
Have fun and let me know how you like Atlantis if you go.