I've never done any scuba diving on the island of Tahiti or Moorea. My only experience in Tahiti has been snorkeling with humpback whales, and in my opinion, that is a worthy experience, whether done from Tahiti or from Moorea. But the season for humpbacks runs somewhere between July and October (their wintertime). Waht is the timing of your trip?
All of the scuba diving I have done in French Polynesia has been in the Tuamotu Archipelago - about an hour away by domestic propeller plane flight. There is a reason why most of the postings in this forum concerning French Polynesia are focused on the Tuamotus, and why the few liveaboards that offer or have offered dive trips are also focused on the Tuamtous.
If you could extend your stay and fly to either Fakarava or Rangiroa (or both), then I would wholeheartedly endorse bringing dive gear and spending at least 4 days in either Fakarava or Rangiroa. Depends on what you wish to see.