If your boat doesn’t have refrigeration, milk for cereal gets “interesting.” For baking and cooking, you can get great powdered milk (read my article about how to find the good stuff). And if you have somewhere cold to keep it after it’s open, boxed milk is a great choice (read about it).
But if you don’t have refrigeration, you really need to find some sort of single-serving milk or you’ll be throwing a lot away. I know. I’ve used just a cooler on several trips, each lasting weeks to months.
Good news: if you do some searching around most large supermarkets (and possibly asking some questions), you can find individual servings of boxed milk and soy milk. These don’t have to be refrigerated before they’re opened and designed to be used all at once.
Grocery stores don’t seem to stock these in one consistent place — even amongst different stores of the same chain. I’ve found them in the cereal aisle, with juice, and even with baking supplies.
Both the milk boxes and soy milk are intended for kids to take in school lunches and often come in strawberry and chocolate as well as plain or vanilla. I can live with vanilla on my cornflakes but chocolate just doesn’t interest me one bit.
Since they’re intended for kids, they come with a straw and a little foil place to stick the straw in. Rather than piercing the foil and trying to pour the milk out of the raggedy hole, I’ve learned to find an edge of the foil and just peel the whole thing off. Much, much easier to pour without it going everywhere (generally including my shorts or t-shirt).
At the store nearest us, they come in 4-packs. And while not cheap, it’s less expensive than throwing away three-quarters of the one-liter boxes. Each box contains one cup — a generous amount for one bowl of cereal or two kind-of skimpy bowls. We generally allow one per person and just drink any that we don’t use on our cereal (if you take milk in your coffee, that’d be another good use for the extra).