I know this is at a slight angle to the question but relates to the relative density question you seem to put. You drown fifteen times quicker in a lake than you do in the sea. besides the sea having more oxygen the salt tends to make the mind get rid of the water or at least try to. For the human which is not aluminium jumping into some lakes or streams you can sink very quickly and so rules are not same for all water that are non sea. Upthrust is probably affected to by currents tht are at angles to that force and which may aid or not. It seems that crashing plane got a good mixture so that the hull was not smashed