I would recommend snorkeling at Coco Cay, Royal's Private Island. The waters are very calm and they've sunk sea planes and parts of a constructed pirate ship to make a reef for your enjoyment. In nassau, I'd recommend staying near the ship or doing a tour with the line. Not a ton going on in Nassau and it's somewhat dirty. You might also want to consider taking a cab (not a water taxi) to Atlantis and walk around and see the hotel - very nice. BTW - the water taxi's to Atlantis are pretty crowded and I also smell like diesel fuel when I get off.
I've also snorkeled at Coco Cay, pretty good and they will go out and feed the fish while you snorkel there, amazing. Just be sure to purchase a water camera before you leave home, you'll save a lot of $ that way.