We didn't book an excursion and found out why we should've. Shuttle bus to the downtown area was a big mistake, area was dirty and not worth visiting. Rain on the way back to the port cinched our decision not to try to taxi to a beach.
"island tour" portion was more like a pressured vacation time share tour, the beach was dirty and the beach was loaded with drying seaweed, plus a constant diesel fumes smell. Carnival DID refund some of the cost to us.