A popular bed for couples stuck in the '70's, usually accompanied with a mirrored ceiling and a deep-seated feeling of shame.
According to various studies, waterbeds relieve pressure on joints and help eliminate dust mites, which trigger allergies.
According to Wikipedia, those are the only two benefits of a water bed. On the other hand, there are 10 disadvantages, one of which is "Homes with many occupants may be bothered by noise pollution coming from the bed, due to sexual intercourse." Amazing.
Also, you get sea sick every time you lie down, which is an added bonus.
My parents had a waterbed, and let me tell you something-- I hated that thing. My earliest memories are of watching TV in my parents room, trying to lie down on the bed and still see the television while staying as still as possible so the noise of the water sloshing around didn't drown out The Facts of Life. I remember when the first one popped and, instead of buying a normal mattress, my father chose to replace the demon bed. My mom eventually threw it out and bought a normal mattress, so there's that, but anyway...
There's a list of things you can't wear while lying on a waterbed. Zippers, buttons, shoes, belts with buckles... Have a pet? Have fun. That bed will last you exactly zero minutes, because animals have some bizzare attraction to things that are easily breakable, and a waterbed is exactly that.
Oh, by the way, this is how you fill a waterbed.
Let's say your thermostat goes out; your bed will be an ice cube, so cold that sleeping on it actually causes pain. If someone brushes against the same thermostat while getting into bed and turns the heat up, you boil to death. There is nothing wrong with a normal mattress. They don't require heat and they don't explode and cover you in stagnant water. Why would you take two really bad ideas for beds and put them together on a bed? There's no reason for this.