Wearing your uniform anywhere other than a military installation is being a target for terrorism. If anyone wants the specific regs they need to consult AR 670-1 and current force condition regs. As I said before, being stationed overseas we are not supposed to wear the uniform even in our cars going to base. Maybe it would be ok if it were an American Cruise, but traveling overseas has a complete different set of rules to go by.
Regulations prohibiting wearing of uniforms ashore on foreign soil are intended to diminish the threat of personal attack by terrorists, street gangs, or organized criminal elements, and thus apply only to areas where such threasts are known or suspected to exist (which may encompas most foreign soil right now). In any case, such threats do not exist aboard any of the ships of any of the major cruise lines marketd in North America so the restrictions don't apply in that situation. OTOH, it would not be advisable to wear your uniform when going ashore in foreign ports.
If you have doubts, though, you certanly can ask your commanding officer to confirm this with higher authority. Be clear, in your inquiry, that you will wear the uniform only aboard the vessel during the cruise.