I assume you're going to boil the water by dumping hot rocks into this makeshift container?

If so, a word of advice: You'll need a LOT of rocks.

Water has a *ridiculously* high specific heat capacity, so whatever weight of water you wish to boil, you'll likely go through a number of times that weight in rocks.

Therefore you need to have a way to both put rocks into the water, and to take them out. Over and over and over and over again.

You could try making the rocks hotter, so each has more heat to give up to the water, but then you just get a lot of steam as the water in contact with the rock boils instantly, and all that "extra" heat escapes.

I think this whole scenario demonstrates just how friggin' important it is to have something, anything, that's capable of holding water.