My girlfriend and I have passports but her 7 year old daughter does not. She just received complete custody of her girl but we have to file a name change and then get a passport but that won't happen in time for a cruise in April from Long Beach to Mexico. I see "Closed Loop" routes do not need a passport only a birth certificate to board but what about getting off of the ships in port? It'd be a bummer to be stuck on the ship looking at Cabo or Mazatlan etc. And yes I'm aware we won't be able to fly home in an emergency but the ship has doctors short of some freak accident.
