Bodies don't always come back up
Here in Ontario, Lk. Superior is known to keep its bodies because cold water temperature prevents gas formation.
Bodies do tend to come up in Lk. Ontario when drowned near shore, but those lost well offshore may not. One sad Spring night several youths took a small overloaded boat out of a marina for a joyride. Not a single body ever came up -- even though the Ontario and New York shorelines were thoroughly searched and several items from the lost boat were washed to the NY shore.
As to the present case, people strapped in in fuselage sections that went to the bottom would be in water too cold for gases to form.
In the cases of bodies that have appeared on the surface, I suspect that they initially sank to an intermediate depth where the temperature was warm enough for gases to form.