On any family trip to Oahu (or other Hawaiian islands), an outing on the water adds an extra dimension. Kids love boat trips, and there's the bonus of great views of the island from the sea.
One easy way to get out on the water is to take a catamaran ride departing from the Outrigger Reef Hotel: a major family-friendly resort, with a convenient location on Waikiki Beach. Kids will love lounging on the mesh and peering down to the sea below.
Other options for getting out on the water include dinner or sunset cruises (make sure kids are welcome), snorkeling tours, Atlantis Submarine rides, and Ecotours.

