How odd, but then cruise line websites are not known for being accurate. I just checked the websites for both the Croatian Embassy and the the U.S. State Department. Both say only a passport is required, and at least for U.S. citizens no visa is needed for stays under 6 months.
I'm like Penny. All I have is a license and passport. When I do consultant work for TVA I have a photo badge for security access to nuclear plants, but that is all.