With the new cingular deal (deal with cruise ship companies) you should be able
to use the GSM phone on the cruise ship (if it has the facilities) but the the rates
should be along the line of international roaming.

To be able to use it, make sure you have your GSM provider enable
International Roaming and call up your provider to check for the charges.

I'm assuming the charges will be bit high.

Also check with the cruise ship company. they may have some rates set
from their end.

If you are near the coast, you can always use the service thats on the
land.

Based on this: http://www.cingular.com/media/roaming_gen

Dont know the CDMA rates, but its also available on some cruises.