I remember wondering the same things on my first cruise. You do not have to book an excursion on the ship if you do not want to. You may get off the ship, catch your own taxi to the beach, or just walk around the port area or town and shop. You can also book your excursions from other vendors besides the ship. Go to this site and look at what is offered Shore Excursions
. I would recommend that you search out all that is available at each port and post as many questions as you want to be well prepared before you get to each port.