I had one in the mid 70s. The attraction was the warmth. I was riding a motorcycle through the winter in New England (excepting when the roads were snow covered as bike street tires are bad at 'clearing' snow so I'd end up with snow jammed between tire and fender. My knees got pretty cold. Knees don't have good blood flow so I'd be waking up with cold (as in icy to the touch) knees. Enter waterbeds. I could dig my knees in.
They disappeared for a lot of the reasons you mention. Last one I saw was just the bladder in the back of a pickup truck on Big Island. I asked the owner - he was using it to turn his truck into a water tanker so he could water his coffee trees (it was a dry year and he was just getting started). No, I didn't try laying on that one. He offered when I mentioned I'd had one.
These days, I'm on 5lb visco foam on top of talalay process latex. Closer to being weightless than on a waterbed (which as I've mentioned wasn't why I had one) Long story but as a teen I used to throw SCUBA gear into our pool, swim down and have a nap - so I have a good idea of what weightlessness feels like. I'm just on the negative side of neutral in fresh water.