We do our own thing when it makes sense to us.
Examples of savings (three years ago):
- Curacao - Ship tour $30/person. We walked off the ship and found $10/person and went to the same places, according to the ship's description of the tour.
- Aruba - Ship tour $60/person to drive in a jeep caravan for a few hours. We walked off the ship and rented our own jeep for $60 total for the day for the two of us. We saw the caravan along our way but also used the jeep to take us back to the beach later.
Examples of when we felt it was to our advantage to book through the ship:
- Cozumel (tender to Playa del Carmen before shipped docked) and bus to Chichen Itza, then ferry back to ship after it docked. This way we were first off the ship and had more time at the ruins. We also had time to shop in Cozumel.
- Grand Cayman (tender). Guaranteed first off the ship by booking the Seven Mile Beach transportation. This was a short day so doing this gave us enough time to do the beach and our own tour around the island.
- St. John (tender). Guaranteed first off the ship. Provided us with clock work transportation: tender boat to St. John, tour vehicle around island, two hours to swim at Trunk Bay, pick up and delivery to ferry boat, bus waiting on St. Thomas side, time to go back to the ship for lunch and tender back to downtown to shop. If we had eaten downtown, we could have probably fit in a swim at Megan's Bay, but saved it for another trip.
I'm sure that we could have done these excursions on our own as well, but not sure that we could have squeezed in as much because we lost no time with transportation connections.
I've noticed that sometimes the ship's tours include lunch or span the lunch hour. Doing our own thing usually allows us to fit in more (tour/beach/shop), providing both a morning and an afternoon slot, with lunch on the ship.
Hope this helps.