I did just speak with RCCL on this same subject for my friend going to the Caribbean. They can you a valid Gov issued ID and a cert birth cert issued by the Gov not the Hospital. However be aware if an emergency happens and he need to flly home I am told he will not be able to. Other than that RCCL tells me it is fine for a closed loop cruise. Me, I wouldn't go without a Passport, worth every dime and good for 10 years. Make sure all documents do match, that is important.