I put the waterbed together.
The top of the side rails of the bed is 18".
The distance from my crawlspace concrete floor to the lower part of the wood supports of my main level floor is 20"!
That means that if I cut off the headboard and the footboard posts, the whole thing will fit in the crawl-space.
I attached the garden hose and it took about 30 minutes to fill. Afterwards, I filled a 5-gallon bucket with the same hose. That took almost precisely one minute. So, the waterbed holds around 150 gallons.
I also multiplied 5' wide by 7' long by anywhere from 6-8 inches deep, converted that to cubic feet, and then that to gallons and got from 130-170. So it sounds like the 150 gallons guess is pretty close.
I believe that water weighs in the neighborhood of 8 lbs per gallon, so that's over 1000 lbs of water! Good thing it's going on the foundation!