Getting between the island groups means flying or taking a boat. The distances are large — the Marquesas are a 3.5-hour flight from Tahiti, the Gambier Islands nearly four hours — and inter-island boats, while they exist, are slow and infrequent on the longer routes.
Air Tahiti handles most domestic flights and connects all the main island groups. Air Moana (formerly Air Archipels) covers some inter-island routes. Fares are not cheap, but the French government subsidises domestic air travel, so they are less painful than the distances might suggest. Book well in advance for flights to the Marquesas and Gambier Islands — seats are limited.
Within the Society Islands, the Tuamotus, and between closely spaced islands, ferries and inter-island boats are practical. A large catamaran runs between Papeete and Moorea every 30 minutes or so and takes about 30 minutes — this is the best way to reach Moorea. The Aremiti and Terevau ferries are the workhorses. Supply ships (the Aranui being the most famous) serve the Marquesas on a roughly three-week rotation from Papeete, combining cargo and passenger capacity — a voyage on the Aranui takes about two weeks for the full circuit and is an experience in itself.
On Tahiti and Moorea, le truck — a local bus running set routes — is cheap and reliable. On most other islands public transport is minimal or nonexistent. Rental cars, scooters, and bicycles are widely available on the main tourist islands; a scooter is often the most practical way to explore. Taxis exist but are expensive. In the Marquesas, horses and 4WD vehicles are the main options for getting into the interior.