Those who haven't might want to consider searching the manifests at Ancestry.com while they are free to see if any relatives made a trip overseas. For citizens, in later years, there should be a reference to their passport on the manifest.
Dean Martin's 1953 entry lists his passport number.
http://www.rootdig.com/ships/famousships/1953_dean_martin.html
Their passport application may provide even more information. Keep in mind earlier manifests do not include passport information, but many after 1907 will list the citizenship status and provide information (brief) on where the person was naturalized.
Just a thought if you are stuck. There are also some lists included of various crew members and others as well.
Michael
