I travel to Mexico occasionally for work. Getting caught with any knife (pocket knives, locking, non-locking, multitools, etc...) will end you in up a Mexican jail. This is not a good thing.

Mexico, Know Before You Go

Whistles, lights, antibotics, anti-diarrhea meds, oral rehydration salts, some sort of water purification, radio, and a phrase book are good ideas. I also usually track down the phone number/address of the nearest American Consulate.

Most people are very nice but crooked cops are everywhere. If one tries to "fine" you insist on going to the police station to pay the fine.

As for the cruise ship, I remember when we took a Royal Carribean cruise they had metal detectors on all the gangways.

-Blast