If you have a current passport, thats all you will need, actually best form of identification you can bring with you. I was actually suprised how many times during the week cruise we needed to show our passports, highly recommend them. You should be fine :-)
If we have valid passports, do we still need notarized copy of birth certificates? We are cruising Western Caribbean -- Grand Cayman, Cozumel, Costa Maya, etc.
No. When travelling within Canada, Mexico, the Caribbean, and Bermuda, U. S. citizens must present (1) proof of citizenship and (2) proof of identity. A passport satisfies both requirements. If you do not have a passport, you generally may use, for example, an official birth certificate (with raised seal) or nationalization papers for proof of citizenship and a driver's license (with photo) as proof of identity.