Untouched waters are the name of the game on the Cocos (Keeling) Islands. The idyllic setting is unlike any other in the world, and you are almost guaranteed to be one of the only few who have dived in specific locales.
Expect visibility of 25m or more and warm, calm waters. A few favorite dives to check out during your stay are Cabbage Patch and the Garden of Eden. Here, you can see stellar corals and an abundance of marine life.
Another favorite is Two Caves, an aptly named swim through. Head to Direction Island, where the diving is exceptional. This island is uninhabited, and you can dive right off the shore.
For diving in the Keeling Islands, fly from Perth twice a week, on a route that goes in a circle from the mainland to the Cocos Islands to Christmas Island.