But huge numbers of tourists kicking around in fins haven't been good for the fish, and today the bay is a carefully protected marine reserve.
This means visitors must jump through a few hoops before getting their toes in the water - but it's worth it.
Visitors pay $7.50 to enter the bay, and must watch an education and safety video before heading down to the sand, an easy 10-minute walk from the ridge-top education centre to the bay, or you can catch a lift on a little safari-style trolley.
Snorkelling gear can be rented, or many tour packages will provide it for you.
My tour driver includes a lesson on using the gear while navigating a minibus along Oahu's coast road. It may not be illegal to drive in Hawaii while wearing a snorkel, but it was a little alarming to watch.
Sea life at Hanauma includes butterfly fish, parrotfish, angelfish and the holy grail of snorkelling here - the sea turtle.
Hawaiian green sea turtles are a threatened species. It's illegal even to touch them.
But it isn't at Hanauma Bay that I see one of the big, gracious swimmers - it is while paddling along Waikiki Beach.
Stand-up paddleboarding is the surfing style Hawaii has exported to the world, so I figure if I'm going to surf here, it might as well be standing up. My one-on-one lesson, on an unseasonably damp, grey Waikiki morning, is with Faith Surf School, based outside the Outrigger Waikiki on the Beach Hotel.
The instructor is worried about the chilly conditions, however he says we'll be okay if we stay pretty dry.
"You won't fall off," he tells me confidently.
I fall off six times. But many of these are because I am distracted by the gorgeous views as we paddle towards Diamond Head.
And one is because I get too excited when the instructor casually waves at something in the water off to our right and says, "Turtle."
Turtle! I fall off. It's just cruising on the waves, its shell about the size of a dinner plate.
Then, by the time I'm back on my board, it's gone.
When we eventually turn back, my instructor - who has been very friendly and helpful - is clearly bored with my slow paddling and, fall off, get on, fall off routine.
As we head into Waikiki, I manage to line up my board on a good-size wave and ride it all the way into the shallows. That earns me a slow nod of approval.
"That was sick."
Yep. I saw a sea turtle, and caught a wave at Waikiki beach. Sick indeed.
CHECKLIST
Getting there: Air New Zealand flies non-stop to Honolulu from Auckland four times weekly with different in-flight product choices: Seat, Seat + Bag, The Works, Works Deluxe and Business Class.
What to do: Faith Surf School is based outside the Outrigger Waikiki on the Beach hotel. Hanauma Bay is a perfect spot for snorkelling.
Further information: See gohawaii.com and DiscoverAmerica.com for more on visiting Hawaii.
The writer travelled to Hawaii courtesy of the Oahu Visitors Bureau, Air New Zealand and the Outrigger Waikiki on the Beach.