You can get to Stone Island on your own, very cheaply. There's a small, local dock, within walking distance of the ship, where you can take a small fishing boat over to Stone Island for just a buck or two. The port area has changed since I was there, so I don't want to give you directions. BUT, I have seen recent posts about getting to Stone Island on your own. It seems to be even closer to the ship than when we went.
When we went in 2001, we spent $5 round trip total (for 4 people!) to get across the channel on the fishing boat, $2 each way for a local to bring us to the beach---again, for 4 people (you can walk though, it's really close), $10 banana boat ride (total for 3 people, I'm chicken
), $20 on snacks and drinks, $4 taxi to get us back to the ship. Total for 4 people: $43.
The ship tour was $70 per person; we saw them. They arrived on a nice catamaran and lunch was included, that's what they paid for. The rest was the same as our experience, except they had to leave at a specified time. $280 versus $43..........