For 30 minute of at port calls it might be cheaper just to roam or buy a EU roam package from your US carrier than to buy a SIM card for EU (Voda EU SIM was about 18Euros) or individual SIMs.

Do you want to spend your precious vacation time looking to buy a SIM card in each port and then try to load it without English instructions? Then you have to give the number for your callers to call you.

Most cruise ship terminal have international IDD coin (Euro) operated phone which is very cheap for outgoing long distance calls. That is how the ship's crew keep in touch with home on their days off at a port.