This event is included in the cost. It is an evening event on a private island. You get fed and then watch a Polynesian show.
For my cruise, they ran exactly three pontoon style boats (much more seating than the tenders) to take people to the event. Then they run three back after the event. There's no coming and going for this one. People could stay on board the ship and order room service for dinner if they wished to skip. As far as I recall, they stopped tender service at about 14:30 the day of the event, so whatever you have planned that day on shore, you'll need to get back for the last tender. I hope I remember that all accurately.
To confuse you more, there's another day with lunch on a private island, with swimming and water sports brought to shore. That one has tenders running during the event so you can come and go to the ship as you wish. That's before you get to Bora Bora.