A water mattress having a portion of its volume occupied by air or gas filled bodies lowers the total mattress weight without sacrificing its desirable characteristics. The gas filled bodies comprise independent, elongate, parallel chambers having sidewall members which are secured to the bottom panel of the mattress. The chambers are positioned at approximately the horizontal midplane of the mattress such that it exhibits the conventional uniform weight distribution characteristics found desirable in such devices. Air is supplied to or released from the chambers through access fittings on the chambers and water mattress. The water filled mattress weighs substantially less than a conventional mattress having the same external volume. Furthermore, the chambers tend to inhibit the sloshing and highly under-damped natural harmonic motion typical of conventional water mattresses.