There are advantages and disadvantages to paying for your excursion on the ship. Advantage you’re covered by the crew ship insurance if you get hurt while on the excursion they pay for your medical needs. Disadvantage it’s incredibly expensive you can get much cheaper excursions after your off the ship and on the island.
You don’t need to go on a $100 per person excursion to have fun once you’re off the ship talk to some of the locals or the shop owners get ideas of what to do. We spent 20 bucks per person in Cozumel which included taxi fare to and from a resort which gave us a free Private spot on the beach with a tropical palm tree like shelter and chairs and it also included free snorkel gear rental.
We spent about six bucks per person round-trip in the Cayman Islands to be brought to an incredibly beautiful beach where we just spent the day swimming in drinking beer by the water. There were food trucks there as well soap, we were able to enjoy some of the local fare.