Welcome aboard, Deborah! I'm glad you're continuing to plan all the little details about sailing with Disney before you head out to sea. Planning is half the fun and can really help you to live more in the moment once you're on vacation, as you know that you've got the details taken care of!
It's really a personal preference for guests as to whether to take their passport into port with them or leave it in the stateroom safe. Do be sure to bring your actual passport to check-in when you first board the ship in Port Canaveral. A copy of a passport is not acceptable.
But, whether to take your passport off the ship in each port or not is up to you. We've done both. When I know the port area well, I feel completely comfortable keeping our actual passports with us as we set out to explore. However, in unfamiliar ports, or when we head out without an official Disney Port Adventure, we have sometimes simply brought a copy of our passport, just to be safe that we won't lose our real one. Since you're setting sail on the Fantasy, you'll be exploring the Bahamas and Caribbean and as a U.S. resident on a closed loop sailing like this, a passport isn't required, though it is highly recommended (see the
travel document requirements on Disney Cruise Line's website). If you're nervous about losing your passport, leave it in your safe and bring a copy of it with you as you go into port each day. It's very unlikely that anything would happen causing you to miss the ship, but it's always better to be prepared. To re-board the ship after your day in port, you'll need your Key To The World Card and a photo I.D.
Have a wonderful trip, Deborah!