Way back in the old threat it was pretty much settled that the bodies sank to an equilibrium level. Then as they bloated they rose if they managed to float out of loosely fastened seat belts. One person claimed that bodies, especially broken bodies, could work their way out of even fairly tight seat belts. He claimed sufficient fresh water experience, if I recall, to make that believable.
No disrespect intended here. Over this thread I have read a lot of your posts and learned a lot about radio and satellites etc. But there is some pretty poor science and plenty of vivid imagination on this thread.
There is no equilibrium level - once you sink you sink. All the way. It is a positive feedback process - progressive compression of gas filled compartments increases average density (or reduces buoyancy), which increases sink rate and so on and so on to the bottom.
This theory would suggest that the body and seat sank to such an equilibrium level (presumably the positive buoyancy of the body negating the negative buoyancy of the seat), and then gas formation created increased positive buoyancy which somehow caused the body to float free of the seat. Sounds like a late night horror movie.
There is some science here.
Bradley Stafford, "The Sinking and Rising of Drowned Bodies" (unpublished thesis, 1988).
"The human body weighs slightly more than fresh water. Consequently, when individuals become unconscious, they sink--regardless of fat level, which slightly increases buoyancy. Generally, a drowning victim will reach the bottom of a body of water in spite of the depth, unless it meets some obstruction on the way down. As the corpse descends further, the pressure of the water tends to compress gases in the abdominal wall and chest cavities. As a result, the body displaces less water as it sinks and, consequently, becomes less buoyant the further down it goes, until it reaches the bottom.
Almost without exception, a corpse lying on the bottom of a lake or river eventually will surface because of the gas formed in its tissues as a result of decay and the action of internal bacteria. This results in reduced specific gravity specific gravity, ratio of the weight of a given volume of a substance to the weight of an equal volume of some reference substance, or, equivalently, the ratio of the masses of equal volumes of the two substances.
..... Click the link for more information. of the body so that it rises. Witnesses to this event have described corpses breaking the surface of the water with force, like the popping of a cork.
In some cases, the body may remain immersed. Extremely deep, cold water conditions (e.g., natural glacier lakes, deep impoundments) may prevent a corpse from ever becoming buoyant enough to overcome the immense water pressure.