I have never been to Pinel Island, but will be going in January. I have done some research on this and have seen pictures posted on the Internet about Pinel Island. Supposedly the snorkeling is good, although I can't vouch for that. However, I did talk to a cab driver the last time we were in St. Martin (2001) and he told me that you can do Pinel Island on your own, for about 1/2 of what the ship wants.
Tell a cab driver to take you to the ferry to Pinel Island. Last I heard, the round trip to the island on the ferry was about $6. When you're ready to leave, take the ferry back and grab a cab back to the ship.