Diving in Tahiti is primarily hard corals with plenty of sharks, rays, and mantas with the occasional group of dolphins. Don't expect soft corals as they are at a minimum there.
Aggressor Tahiti is big on drift dives through passes into the lagoons of the atolls they visit. This also involves "hooking in" as they like to stop the drift and watch some of the marine life as it travels with the current. They also do some corner drifts. On a typical trip you will dive the atolls of Rangiroa, Fakarava, and To'au. Vis is usually very good and water is warm, but weather can sometimes be an issue...depends on time of year.
I'd make the trip for at least 2 weeks. One on the Aggressor and one for other high islands before, or after, the liveaboard. Bora, Mo'orea, Huahine, and Tahiti all have land based operations that can provide additional opportunities to dive.
hope that helps
12v