Oh boy, I can imagine your excitement.
Others will have to fill you in on excursions that include snorkelling. I have only done two excursions that did include some. The Doctor Poole dolphin expedition is outstanding, if you like wildlife, but be prepared to examine every can of tuna before you buy it, forever after. We did some snorkelling on this excursion, and I thought it was very good.
The other excursion that included snorkelling was the shark and ray feeding in Bora Bora. It was great fun, and there were some good reefs out there.
These two excursions both had snorkelling that was better than the two motu sites that are included for free, although both of these are still great fun. On the Bora Bora motu there may be some better snorkelling than I found, but you have to beware of currents there. The Tahaa motu snorkelling is just so-so.
Other excursions I particularly enjoyed: 4x4 trips on Bora Bora and Moorea; land and river trip on Raiatea. Next time I'm going to try some other snorkelling adventures, if I can persuade my husband--he's not as keen as me on the water.
Have a great trip, and tell us all about it when you come back.