Rent a car and drive a little - e.g. to Hanauma Bay. One of the trolley lines (blue line) goes there, too. The problems are, it´s crowded, there´s not much room to swim, they charge an entrance fee - and don´t even try to use the restrooms down there.
Everyone goes snokerling in Hanama Bay in Honolulu. It has become so bad that they charge for entrance and have gated off certain areas. You can't even feed the fish anymore. Keep traveling down the road around the island, and you will find some great beaches that are less crowded. Plus, there is one of the best body-surfing beaches on the island just past the Bay. Finish off the trip by driving back through the Pali Cliffs. Amazing scenery.
No one on VT would not recognize Hanauma BAY would they? Due to just too many people too much of the time, this beach is beginning to show signs that are disturbing to the oceanologists and marine biologists. It would not surprise me if more restrictions were placed on this former volcanic crater.
The vast majority of the people stay fairly close to shore. If you venture out past the lighter green water and get to the blue water it is not uncommon to see large sea turtles. Ann had such an experience and went for a ride by hanging on to the front of the shell near the neck. The turtle did not seem to mind but she became just a dot when she finally let go and started swimming back in to shore. Growing up on the beache of Carlsbad in Southern California gave her an edge. It is not recommended. even if you are a good swimmer.