I would hop in a cab and head straight to negril. Pick one of your favorite areas of the beach to hang out at, swim etc. Order drinks and food and maybe even pick up some Biggas jerk chicken on the way back to the ship. Way better than Mobay in my opinion (and I have stayed in Mobay). Just get a good driver who will make sure to get you back to the ship on time. Ditch the cruise bus and people for the day.