I spent a week in Negril and it's just my opinion but I am not going to spend two and a half hours minimum round trip on a bus just to go to a beach and have lunch when there are lots of nice beaches much closer.
Dunns River Falls is always far to crowded for my taste.
I highly recommend Rafting on the Martha Brae river. It is very scenic and very relaxing. Carnival offers the trip or you can book it directly through the website with the link I am posting below. Only about 30 minutes each way to that excursion and you'd probably be back in Mo Bay in time to take in a beach before going back to the ship if you want to.
http://www.jamaicarafting.com/
Never done the Pirate excursion in Grand Cayman. We have done Stingray City and Seven Mile Beach and enjoyed both.
