All the islands have excellent scuba diving and snorkeling, and all but a few of the resorts have their own dive operations or can easily make arrangements with a local company.
The lagoons in French Polynesia are known less for colorful soft corals than for the wide variety of sea life they contain. Both the number and variety of colorful tropical fish are astounding. Stingrays and manta rays are prevalent, and some in the Society Island lagoons are quite friendly to humans, the result of having been hand-fed (you'll see them hanging around the waterside restaurants after dark, hoping for handouts).
Rare is the diver who doesn't encounter sharks here, though most will be of the relatively harmless reef varieties -- blacktip, silvertip, whitetip, and gray. The most visited islands now have shark-feeding encounters. Hammerheads and other large sharks live outside the reef and in the passes leading into the lagoons.
Most dive sites are along or just outside the barrier reefs and within short boat rides of the resorts.
If you like to snorkel, you're in for a few treats, whether on a shark-feeding excursion with other tourists or just swimming off your hotel beach. In some cases, snorkelers can go out with scuba divers.
The best diving and snorkeling are at Rangiroa, Tikehau, Manihi, and Fakarava in the Tuamotu Archipelago, where the huge lagoons harbor an incredible variety of fish and sharks. Go to Rangiroa to see sharks; go to the others to see more fish than you ever imagined existed.
The atolls are also home to heart-stopping "ride the rip" dives and snorkeling trips, on which you literally ride the tidal current through the passes into the lagoons.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.