A girl going with any guy who is a stranger is just crazy.. even another girl she dont know is odd... Only way anyone should go with a stranger is if they get each other's facebook asap.. and see them and how they interact with others. Give it a few weeks or a month or 2 in communicating and checking them out..maybe even doing a background check is good. Then if everything seems fine.. then take the risk if you want.
Sure it's just a roommate but you still are living with them for 4-7 days depending on your cruise.
A guy 64 wanting to share with a girl in her 20s is just creepy. Sure we all would love to room up with a girl in her 20s at that age, but lets be honest, if you were just looking for a cruisemate to cut prices, another guy, or someone late 40s-70s is fine. Infact over 70 would be good for you since they will probably be in bed by 10pm.