Hawaii is made up of seven islands. The Big Island of Hawaii, Kahoolawe, Maui, Lanai, Molokai, Oahu, and Nihau are separated by rough channels which make them hard to cross by boat. There are a couple of ferries that offer service between two of the islands, but otherwise, most of the travel is done by a plane or a cruise company. For those who wish to explore the individual islands instead of staying on just one of them, these options offer the most reliable form of inter-island transportation.
Travel by air if the island you want to stay at is unreachable by boat. Four main airlines have inter-island service flights: Island Air, Pacific Wings, Mokulele Airlines and Go! Airlines. Sometimes these companies will hold fare wars, which will drop the ticket price. It's possible to get a pass for however many days a person might stay.
Ride a ferry to visit nearby islands. Service is avaliable between Maui and Lanai as well as Maui and Molokai. The two main ferries are "Expeditions" and the "Molokai" ferry.
Take a cruise if you wish to see all seven islands and spend a little time on each. This is a great option for those who are visiting Hawaii for the first time.