Despite the fact that this harbour is still being used by the US Navy, it still has some good sites that are worth seeking out. Waters are probably as clear any harbour in the Pacific and some of the corals are still in good condition, or are regenerating.
Gab Gab 2 can be reached from shore after a 20 min swim but as you have to access it through the military base, a boat is probably your best option. It's a popular site and the reef is in decent condition. You'll come across batfish, jacks, octopus and turtles here. Depths range from 12-40m.
Western Shoals has seen better days, but is slowly coming back to life. The reef is deep in places, reaching 35m and has plenty of crevices where interesting marine life can be found. The shallows, at 5m are ideal for snorkelling.
There is a good stony coral formation on the south side of the harbour. It speads out fingers, hence its name, Fingers Reef. It can get quite deep, but most of the fish action is at shallower depths.
Finally, there's Hidden Reef which is at the mouth of the harbour. Coral is in good condition here and fish life prolific. Expect to see butterflyfish and many other species, including the occasional hammerhead shark that patrols by.