If you take a boat trip along the Na Pali coast of Kauai and the seas are calm, you might get a chance to snorkel in one of the many sea caves along the coast.
Zodiac Shore Excursion along Na Pali Coast of KauaiPictures from Haleakala Volcano on the Hawaiian Island of MauiMaui - A Day at the Haleakala Volcano
Hawaii CruisesZodiac Raft Trip on Kauai from Go Hawaii GuideTouring the Na Pali Coast of Kauai