A test was recently done by MYTHBUSTERS, a TV program, that surrounded a runabout powerboat with a network of pipes that gave out a lot of gas, and bubbles, causing the water to lose its ability to float anything. A boat floats because of the water it displaces. When it cannot displace 'solid' water, it will cease to float. If a gas eruption happens under a large steel ship, it would instantly sink, taking everything with it. But htis does not answer the problem of loose floating objects like lifejackets etc. They may be sucked down but should eventually rise to the surface. In the case of some ships, nothing has come up.