If you had a pot to shape it over, why use the foil?

I think the problem comes in with the folding. I tested this by folding a piece of heavy-duty foil, 18 in square, the same as I would in my PSK, then unfolding it and shaping it into a water container. I had a leak. Presumably this was caused by the folds.

But I still keep foil in my PSK because you can use it for cooking or as a reflector for your fire, and you can patch it with duct tape where it leaks and use it to hold water. And if you heat the water by dropping hot rocks in the container, I bet you could boil water with the duct tape patches in place.