Leon wrote:
Have you checked a water bed store? It is shockingly surprising how little
wood is used and how flimsy all of them are until you add hundreds of pounds
of water on top to stabilize the whole structure. Additionally I am shocked
that each time that I set my sister in-laws water bed up that it continues
to hold up. I know that the bed has been moved 6+ times in 20 years. Hers
is a king size with 12 drawers.

Ditto that. I had a king waterbed, a Sealy Hydropedic (best night's
sleep I ever got, BTW) that I had to give away, and it was amazing to
me how chintzy the frame was. On the other hand, I've seen a load test
of a red oak 2x2 and it supported just over 9,000 pounds before it
failed. Even if your waterbed is 10x10 and a full foot thick, that's
6,300 pounds or so. Obviously, there are lateral loading
considerations for motion in the ocean, so to speak, but 2x lumber
frame finished out with 1/2" ply would be more than sufficient.

-Phil Crow