Abstract: A portable inflatable water displacing body for preventing complete submersion of watercraft in immanent danger of sinking due to a sudden influx of water is comprised generally of at least one bladder, normally mounted within the hull of the vessel in a collapsed and deflated condition, each bladder having: 1) internal volumetric expansion means including an inlet connected to an external gas source for charging the expansion means and a relief valve for controlling the amount of pressure in the expansion means, and 2) a check valve for allowing one-way passage of ambient air into the bladder. The expansion means is comprised of an extremely light-weight, highly flexible tubular framework which when charged with the gas expands into a pre-defined three-dimensional geometric shape causing the volume within the surrounding bladder to increase. The increase in volume reduces the pressure and creates a partial vacuum, which is filled by air pushed in through the check valve by the higher atmospheric pressure.