[inquisitor]

Another interesting SIM card for roaming in Europe is toogle mobile who offer very low voice rates in all those European countries from which they offer local numbers that can be activated free of charge for 30-day periods (so-called "toggle countries"). With toggle you could receive calls free of charge and you could call home to the US for just £0.03/min while you get a signal from German, Danish, Swedish or Polish networks. And once you sail down to the Med you would also get these low rates while you pass Dutch, British, French, Spanish and Italian waters (after you've activated local numbers from these countries which is all done easily online before you depart).
Toggle have had plans to offer cheap data bundles for all those "toggle countries" for quite a while which may become a reality before you board the vessel.