Carnival will provide a beach towel for you to use everyday and you can use it for the pool or excursions. They will leave it in your cabin along with your bath towels. At the end of the day just leave it on the floor and when they come in at night to turn down your bed they will leave you a fresh one for the next morning. There is no need for you to bring your own, unless of course, you think you will lose it for sure! They do charge for lost towels.
