I know this is too late for you as your cruise has come and gone. however, if anyone else is needing this same advice, consider bringing your own milk and having them store it. Horizons makes a shelf stable organic milk in individual serving sizes. You could store that in your room.
I think you already got the info you need on heating it. The easiest way is to plop the bottle into warm water. Tap water should be warm enough or you can ask room service for tea water.
Do not forget a small supply of dish soap for keeping the bottles clean. A sponge or a bottle brush may come in handy. You may want to bring the bottles with disposable liners to make clean up easier.
And as a side note, the world average weaning age is 4 years old. Americans just wean much earlier than most every country. If your children feel more comfortable with bottles while traveling feel free to give them a bottle. Of course it is a good idea to brush their teeth after to remove the milk before bed. Your children will wean before going to kindergarten!