depends on where you are flying too, and from. North America to europe can be very substantially cheaper, especially for Premium Economy or First/Business Class. The same can be said going to Asia.
It never shows on the deck plans, and it can and does change depending on the number of people that they expect to attend. We have actually seen it change locations mid-cruise because of the numbers
We did the Moorea Sail last November....and yes, they supply fins and vest. I should warn you however, that the snorkeling area is a long swim from where the boat anchors, and the current was very strong the day we were there.