An inflatable swimming pool relies on the watertight bottom to hold shape. Without a sealed bottom you're building a flood barrier. You need something that both molds to the shape of the surface, and is heavy enough to provide the downward force to prevent water from going under it. Inflatable flood barriers exist and are filled with water.

I guess you wouldn't really need it to be extremelly watertight, just tight enough that the hose puts more water into it. Seems like it would pay for itself with 30k gallons needed