Babs,
We snorkel but do not scuba. We did meet another couple that did the pool lesson and beginner dives on board the PG. They had wanted to do certification at home in Colorado but she had experienced exercise induced asthma (for the first time in her life) about the same time and could not do the dive lessons at home. Once they were on the PG she had to go to the ship's doctor where she checked out fine and they did their lesson in the pool and then had some wonderful dives.
We did not do the "AquaBlue" but like "ChatKat" Kathy experienced, everyone we spoke with who did it loved the experience. One guy wrote in his trip report that it was his favorite excursion. If you wear glasses you can keep them on - your hair does not even get wet. We saw them doing that excursion when we were snorkeling in Moorea because the two excursions are done in the same area. Those "AquaBlue" participants only had about 3 to 6 feet of water above their heads so there really is nothing at all to be afraid of. I would guess only if you had a problem with claustrophobia or dislike of being slightly underwater would it be a problem.
Are you missing anything? Yes, if you are not planning on going over on the free tender to the motu on Bora Bora for at least part of one of your days there. Unlike Motu Mahana at Taha'a there are no umbrellas, chairs, rest rooms or buffet lunch but there is a large beautiful beach shaded by palm trees, scenic views in every direction, lagoon for swimming and snorkeling and a free full bar. On day 2 at Bora Bora it is a short tender ride away. You could go over am or pm depending on which part of the day your shark & ray feeding excursion will be. Friends of ours went over with us for most of the day on day 2 but they had also gone on day 1 in the afternoon following their morning wave runners excursion. The ship's photograper is over there day 1 in the pm and he got great shots of people over there then. Day 2 is his afternoon off. Many of the crew live for their free time over on that beach.