If you have not already done so, you can go to the "Picture Links" portion of this bb and see our photos titled "French Polynesia: Paul Gauguin & Manihi Pearl". I included some photos of both Motu Mahana at Taha'a and the Bora Bora Motu. Your wife would have a wonderful time at either beach and you could easily find someone else to snorkel with since there would be so many other PG guests there. (Many more at the Taha'a motu than the Bora Bora Motu.) Be sure to order a box lunch the day before the Bora Bora Motu since there is only a full complimentary bar provided on the beach. Many people have spoken about taking your own air mattress to have on the Bora Bora Motu, but we were just fine with using our beach towels on that sand.
We never did more than 1 excursion per day. The days that we went to the motu's that was all that we did off the ship. Some people rushed to do two excursions a day but that would not be a relaxing vacation for us.