When we cruised in November, all we brought were our passports. DH and I had our driver's licenses as well, but a passport was all we brought for the kids; BC's stayed at home.
I made two copies of the pages with our pictures/information and the passport number. One copy remained at home with my sister in the event of an emergency; the other copy we carried with us and made sure it remained on the ship. If we left the ship, we carried our passports...but the copies were in our room safe if they were lost or stolen ashore for easy replacement.