The ship's tours are very good, and identical to many privately-offered trips. I can't comment on private tours, since I have never done any.
I don't remember any drift-style snorkelling trips on the 7-day, although maybe I just never did one. If you are a good swimmer, and an experienced snorkeller, there is drift-type snorkelling off the Bora Bora motu--the current runs between the motus, and can be quite strong, and the water is deep.
The shark and ray feeding on Bora Bora includes some coral garden snorkelling. In this, the water is fairly shallow, and you just swim up to these big coral outcroppings. Swimming over them is kind of scary, since you don't want to touch them, so I tended to swim around the edges,and between them. Very pretty. No current really.
The stingray feeding, on that excursion, and on the Moorea version is alot of fun, not sure which I liked better. The Moorea "lagoon discovery with snorkel" (I think), includes this, and some simple snorkelling with no current--other people have loved this, the fish are great, but I felt the coral was not in very good shape.
