I can barely keep afloat and as a first time snorkeller ,I managed to do quite a bit of snorelling at hanauma bay. Actually we could see shoals of colorful fish while standing in knee deep water.
The water is wonderfully clear and snorkelling was very rewarding , even for a beginer level swimmer like me.
The bay is mostly protected by a coral reef wall -like structure, so the impact of waves is mostly kept out. Some parts of the water do experience waves, but you can talk to a lifeguard and they will direct you to easier waters.
The expert swimmers can go further into the sea, and I guess that is where you can see the turtles and all.
The snorkelling equip can be hired for 7 dollars each set and if wear glasses, you can get an optical mask set for about 10 dollars.
Entrance fee is dollar 5 per person. You can hire lockers, I dont exactly know for how much though.
But make sure you get there very early. If you are driving , the parking lot fills up REALLY early!
Have a nice trip!