water bed

from WordNet (r) 3.0 (2006)

water bed
    n 1: a bed with a mattress made of strong plastic that is filled
         with water

from The Collaborative International Dictionary of English v.0.48

Water bed \Wa"ter bed`\
   A kind of mattress made of, or covered with, waterproof
   fabric and filled with water, thus providing uniform support
   to the body lying on it; -- also a bed constructed so as to
   contain such a mattress. It is used in hospitals for
   bedridden patients, as well as in some private homes.
   [1913 Webster +PJC]