Get a trash bag that is the same size as or bigger than the container. For example, if you have a 30G brute trash can, get a 30G Glad plastic bag like normal people would use for a trash can.
Put the trash bag in the trash can. Pour your water in, including whatever else- rocks, animals, etc. Close the trash bag up and tie a knot. The trash bag is your vessel, the can is just there for structural support and to prevent holes.
I do this all the time with coolers, trash cans, heck you can even make a duffel bag into a water container if you wanted...