I have visited this area several times, both with large cruise ships and smaller expedition ships.
Bora Bora is an evolving atoll, with a ring surrounding reefs and islands encircling a lagoon, with very mountainous island in the middle. The scenery is very dramatic. It matches the classic image many people would have of Tahitian island paradise, of blue seas surrounding steep jungle covered mountains with sandy beaches. If you are arriving in a cruise ship, it will be spectacular, as you are viewing from out at sea, and you have seen mainly flat ocean for much of your distance.
Being out in the tropical ocean, obviously one of the main things to do in these areas is to swim and snorkel around coral reefs. The snorkelling in the lagoon is OK, nothing special. However, one of the fun things to do is go on an organized tour where they feed the rays and sharks while you are in the water with them. This is offered by a few different operators in the lagoon. The sharks are reef sharks, but they are still intimidating. This feeding is usually done in deeper water. The feeding of the rays is usually done in shallower water where you can stand. The rays seem harmless, but it is oddly disconcerting when they come mooching up to you and flapping around and making sucking noises with their strange underhung mouth.