the AT&T cellular data plan can it be used outside of the US for example when cruising internationally>

You would need to speak to ATT&T and see if they offer roaming service in the interested areas.

That is entirely dependant on the type of Plan you have. conatct AT&T for details and costs for Roaming plans.

International data roaming is usually quite expensive. It will probably be less expensive to get pay-as-you-go data plans from European cell carriers. Or ask if your cruise line and the places you'll be saying offer WiFi.

Regards.

Also, check with the Cruise line many newer cruise ships offer wi-fi service for their passengers, some with small added fees others for free.