Kaneohe Bay is the only true barrier reef and lagoon present in the main Hawaiian Islands, 7 miles long and 3 miles wide.  Numerous patch reefs dot the green waters of the lagoon with an average depth of 40 feet.  The bay is dominated by finger and rice corals, seaweeds, and sponges.  It is an important nursery for marine life including Scalloped Hammerhead sharks.   The Hawaii Institute of Marine Biology is located on Coconut Island (Moku o Loe).

Snorkeling may be done anywhere the water is clear enough while scuba diving is best beyond the surf line.