How safe are they?
An electrically heated water bed sounds like a recipe for disaster.
In the 35 years since I bought my first one, I’ve never heard of anyone getting electrocuted. That would be quite a feat. First, the vinyl bladder has to leak. The electrical insulation of the heater has to have worn through to bare metal. That’s a trick because the heater lies pressed down by a few hundred kg of water. Finally, the vinyl “tub” the mattress sits in (in case the mattress leaks) has to have leaked, too, in order for water to get to the heater. Even then I think the odds are slim because it’s just a big resistive element. I suppose if both wires some how got exposed. Did I mention the several hundred kgs of water lying on it?
IOW, file it under “Things I Worry About When I am Done Worry About Killer Bee Attack’s”. It ain’t gonna happen.
That said, they use electricity (you will not sleep long on an unheated water bed), take a small bit of maintenance, and keep the dogs and cats off it. I'm happy with our memory foam mattress.