If yo uhave not seen it yet, see the details on this page for AT&T and princess ships
http://www.wireless.att.com/travelgu...=1633&MNC=CING
It may help. It says you don't need international plans for voice access, but will be charged international rates, but does not say if it is the same for data.
The actual provider of the service on Princess has their page here
http://www.mcp.com/princess/
it may be easier to just contact them and see if they can tell you any options, again they are quite vague on their website.