Snorkeling conditions can change from week to week depending on currents etc. I know this from our recent experience on a back to back. On week one the visability at Motu Mahana was not great, but on week two it was fine. We were on the first tender over each week but made sure that we immediately got into the water for our snorkeling the second week (before other swimmers and the kayaks could stir up the water).
Wendy had a great experience with Dr. Poole, but on our excursion we saw NO animals, and due to the fact that we had to spend the entire time checking each pass looking for dolphins, we did not have an opportunity to snorkel. (With Dr. Poole's trip snorkeling is a possibility but not a given.)
We only made it to the private beach in Bora Bora once. I had previously read on the boards that the currents were strong there and the snorkeling difficult. We had a beautifully calm day and wonderful snorkeling. Ask for box lunches the day before and take them over to the beach with you. It was our most beautifully relaxing day of the cruise!
Our favorite excursions that included snorkeling were the "Shark & Ray Feeding" on Bora Bora and "Lagoon Discovery with Snorkeling" on Moorea. This second tour also stops at an area where you can pet and feed sting rays, then goes to a coral gardens area near a beach. It is best suited to experienced snorkelers due to some of the snorkeling being in shallow water. Bill and I had no problem, but two men were cut on the coral. A photographer is the second crew person on the "Lagoon Discovery..." on Moorea and you have the opportunity to purchase a VHS tape or DVD of your excursion. We were very fortunate and spotted both dolphins and whales (first week of September) on our way back to the dock so we have our DVD with those images as lasting memories.
We took our own gear but did borrow mesh bags from the water sports platform to hold our gear. It was checked out for the entire 2 weeks. Borrowed gear does need to be returned Friday afternoon. You can not snorkel from the water sports platform. I think that changed following 9/11. But I believe that they were open every day except for Papeete; and this is where you sign up and use the kayaks, windsurf boards and go water skiing.
I'm sure you'll have a wonderful time. Like Wendy, I'll be waiting to hear all about it when you return.