My husband and I visited Hanauma Bay while we were on our honeymoon on Oahu. We got to the bay using the Go Oahu card which included a shuttle from Waikiki (where we stayed in an Airbnb) to the bay. When you get there, be prepared to watch a short film about keeping the reef safe. It is entertaining to see people whine about having to watch it. The good thing is that you only have to watch it once a year. So, get stamped as you leave so you can come back on the same day or register with them so you don't have to watch it again for a year.
There are lockers you can use but I really don't think your stuff would get stolen off the beach. There are many people around and lifeguards so anything shady would get noticed.
Definitely wear or bring a swim shirt, the water is actually pretty chilly and we had to take several breaks from snorkeling to warm up.
We saw a seal, an octopus, sea urchins and so many fish! Although there were lots of kids there, I wouldn't recommend this place for young children, there are lots of sharp rocks and coral that could be damaged if someone who didn't know what they were doing messed with them. Also, even though the bay is protected, the waves could get pretty intense and pushed us into the rocks a couple times.