Hi, KimK! I was on the PG in June and knew a couple having their vows renewed. It was done the afternoon of our visit to Moorea, up on the top deck near the Bar du Soleil (wasn't open). It was by invite only. They had a small cake, champagne, flower leis and headpieces, the traditional ceremony by the Captain and the polynesian portion of the ceremony with the cape/blanket, and photos by ship's cameraman, with EmCee duties by Cruise director - about 15 guests. Very nice. Lasted about 15-25 minutes. It's a package deal that you can arrange through your T/A (best in advance so the ship can have cake and flowers ready). Once you get on board, you should talk to the cruise director or social director (Should be Claudia Perriou). The backdrop alone is drop-dead gorgeous.
They also have an anniversary ceremony for all couples one afternoon by the LaPalette Lounge, aft, where they do similar ceremony for all, then have individual photos with borrowed cape/blanket and flower crowns. I don't believe this one cost anything, but the other individualized one does have an extra cost. Do Both!!