Although most cruise reviewers say there are five, there are strictly speaking only two cruise lines that truly have a "ship-within-a-ship". A fully self-contained world where passengers paying at least $500 per person a night for a suite are given the special entry card to a door that lets them step out of the regular cruise ship and into an exclusive VIP (Very Important Passenger) one. I have stayed on all 5 of them though and review and rank them.

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