Robert A. Heinlein invented the water bed, describing it in Stranger in a Strange Land in sufficient detail that later attempts to patent it failed because it was made public domain by being part of a published work. I think Heinlein received a free water bed from the first person to manufacture them.
Heinlein saw the water bed primarily as a hospital bed, having thought of the idea while he spent months in hospitals recovering from pulmonary tuberculosis in 1934.
Update: gn0sis points out that given the fact that even Webster 1913 knows about the water bed, it is unlikely that Heinlein invented it. However, "Heinlein invented the water bed" is a powerful meme on the internet, so I'll let it remain here. Just take it with a grain of salt.
General Wesc says the first water bed dates back to 1871.