If you got a softside, it would weigh 1133.86 lbs., and measure 65"x69"x7", so you would need 3. That's a lot of weight. I'm not sure if you would need to fill completely. Also, they are not sun resistant, I'm sure. So you would need to put something over it. And, now i was thinking that waterbeds don't seem to put out any heat into the room, they retain it. So, bad idea, sorry.
