Arguably the best scuba diving in the world is in the Cayman Islands. This unique place has crystal clear waters, reef systems that are very close to shore, mild currents, white sand beaches, and abundant marine life. The island has no taxes, no unemployment, friendly people, and virtually no crime. What's not to like?!
Deadly Puffer is an area named after a legendary blowfish that supposedly attacked Scuba divers invading its territory. The Blowfish may be gone, but some of his friends are still here, such as Stingrays and schools of fish.
Other sites worth seeing on the island are Stingray City, where wild (but tame) Stingrays eat out of your hand, the Turtle Farm, and the beach at Rum Point and the 7 Miles Beach. Another interesting site, although not worth paying extra to see unless you already have a rental car, is the "town" of Hell. Basically, it is a small area of unusual weathered limestone formations. The town got its name about 100 years ago when a visiting minister from England was bird hunting and shot a bird that landed in the limestone (which made it inaccessible). The minister blurted out "oh, Hell", and the name stuck. Other than spending 1-2 minutes looking at the limestone, the touristy thing to do is send a letter to friends from the post office, which will be postmarked from Hell.