JD-EE, thanks for the link to the old thread. However, on reading Kulwin Park's post, he does not really say anything about bodies slipping out of seat belts. I now consider this a somewhat doubtful possibility, except where accompanied by seat or belt failure.
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Also, there are new and old comments about bodies sinking and later surfacing due to gas build up. I expect that this scenario is limited to the common case of shallow water. Once something sinks in several thousand meters of water, it would take a LOT of gas to make it buoyant again,, due to the high ambient pressure. Something that is very slightly negative can reach equilibrium in mid-water (water density increases slightly with depth), but that is hard to arrange on purpose, and not likely to happen by coincidence.