Hi
You probably could but wouldn't it be rather small??
What did you intend to keep and what depth?? gary

Gary, I was thinking to use it for fish but after your comment it might not be deep enough? Would a rain garden work? I do want it to be small, the mattress is apx 7' x 5.5' and was hoping it would work.

Thanks for making me think! plantbug

Hi
If you want a small pond think I'd skip the fish and go for a marsh ,bog rain garden type setup concentrating on the various types of plants In your area a good choice might be the various types of carniverous plants that not only need a wet situation would opppreciate your winter temps . For a little more depth emergent swamp type plants ,a gazillion to choose from. For true aquatics you generally need at least 2 feet ,as most of these types need to float also allows them to cope with winter. This would be the situation for smaller fish .
Oviously the size of the liner would dictate the size of the pool and the choices of types of plants/ fish.
Sounds like you're thinking wet area not necessarily a pool. Given any thought to filtration, water features circulation?? Check around at the gazillions of approaches water gardening will help clarify more precisely what you want?? gary

I would wonder about UV resistance if you used the water bed mattress.