The Great Barrier Reef is the ultimate dive destination. The world's largest reef (it can be seen from space), this haven for serious divers sits off of the coast of Australia. Once underwater, divers will find that the reef lives up to its reputation, with thousands a species of fish and crystal clear waters that make seeing all the wildlife easy. Because it's so popular, dive trips here aren't cheap.
Also, in an effort to conserve and protect this vibrant but fragile place, boat traffic is limited in certain areas and fees are levied on companies that operate around the reef (with much of the money used to fund conservation initiatives). Most dive boats support the conversation efforts by using permanent anchor points so that they do not drop anchors on the reef itself.