If passports were required in any of the ports, all passengers would be required to show their passports before embarkation on the first day. When we went through the Panama Canal on the Wonder, a family thought that passports were only required for adults, not their baby who was less than two years old at the time. They had to have a non-cruising family member rush to the house and get the child's birth certificate and fly it to Florida, so the parents could apply for an emergency passport. IIRC they missed embarkation but were able to join their extended family on the cruise later.
Basically, if your I.D. will get you onto the ship, the same I.D. will get you back on the ship at any of the ports.