We have been waitlisted 3 times before, and the only one we did not get when we got onboard was the Dolphin Encounter. As I recall, though, we did something else and had a great time, and our friends that went on the Dolphin Encounter thought it was disappointing because there was so little time with the Dolphins. Another thing to consider: Princess may also add another group, depending on the excursion, and depending on demand, so it is worth hanging in there!
Good luck!