I grew up in Kailua on the beach about a mile and a half from KMCAS - towards Lanakai. Snorkeled and dove it every since Nemrod invented the Snark line in the 60s. You know, all along the reef there is actually pretty interesting diving and the shallow water lets you stay down forever.
I don't know if you'll be able to figure this out. There used to be a rifle range on the base that shot into sand dunes. Towards the ocean. Ok if some of the Marines aimed a little high and the bullets went into the water, that area would be called Zombies, by local surfers.
Down in the coral of the reef there, are hundreds of lobsters. Now the Marines might try to chase you away, but if you swim underwater from a quarter mile down the beach they won't know you're there and you can see the lobsters. Little red boats are game wardens, seeing one of those you better not have a glove on or they'll think you're poaching, which was always true in my case. How I wasted my youth.
Or if you don't want the excitment of baiting Marines, just park at the Lanakai point boat ramp and wade in and snorkel around out to the litle island that we call flat island. Then head NW along the reef, going down on air if you see something cool. Beware of idiots on jetskis. There's always a brisk windward breeze and you will drift quickly towards the beach if you stop swimming. Under water, the current tends to pull/push you into the coral. Never a problem in the old days as we just grabbed the coral and held on - can't do that anymore.
Wear a 2 mm shorty - I always dove all day with nothing on but shorts (and at night naked) but usually came out of the water shaking like a paint mixer. The temp is about 75 all year I think.
The water is safe - I made many solo dives even at night - short though because lights usually leaked in those days. It takes extra stupidity to hurt yourself in shallow open water. Bring a sharp knife! Fishermen lay illegal gill nets at night and those things can kill you if you get in one below the surface with no knife. Never saw a shark, although one killed a friend Billy Weaver in 1958 off flat island.
That's about it - this is not great diving but kinda fun. Kailua beach is a very nice stretch of sand, though euro women have started hanging out topless last few years - you do NOT want to see them, trust me. Vis is marginal, maybe 40-50 feet - it aint no Cozumel but it's fun if you live nearby and have a supply of booze.
That's everything I know. Avoid Kaneohe Bay - it's muddy and offers only clam mucking during a short season.
