Hey there, Jean! Welcome, and thanks so very much for cruising on by with your question. We're so glad to help.
It's not just a
Disney Cruise Line thing, but any cruise that leaves and returns to the same United States Port is known as a "closed-loop" cruise. It's part of the Western Hemisphere Travel Initiative, and it basically was established by the US Government to help people know what documents would be needed when they travel.
Your Disney Wonder sailing in April is one of these closed-loop cruises. You'll sail from
San Diego, and you will return to the same port in San Diego. Because you begin and end your trip in the same US port, your
required travel documents would be either a passport or another government-issued ID (like your driver's license or REAL ID) and your birth certificate.
Jean, some cruise lines require passports regardless of the itinerary, so perhaps the person who told you they needed their passport was on a line that required the passport. It's also possible that the cruise didn't end at the same port where it began.
If we can help you with anything else, please come back and see us!
Anna