What you need to figure out is how high up the waterline can be before the vessel begins taking on water (or at a height you feel is acceptable for water to be incase of rough seas).
Let's say you want the waterline to be at the extreme and be right at the top of the perimeter of the boat (aka, a simple boat). What you do is you find the volume of the water that is displaced by the volume of the boat below that waterline. For example, if you have a cube shaped boat and its dimensions are 1m x 1m x 1m and when the water is 1m high, you consider it capsizing, the boat can displace a maximum of 1 cubic meter of water. If you have the water at sya, 0.5m high, it is only displacing 0.5 cubic meters. Now you need to figure out how much 1 cubic meter (or for the second example, 0.5) of water weighs. That way is how much of a load your cubic boat can support.
So in general, the boat can support as much as how much water it is capable of displacing before going under. For your milk carton example, it can basically support the mass of 2lt of water (minus the mass of the carton of course :) )