Question

why a dead body floats in water with its head immersed in water.

Answer 1

I already answered this, but anyway, have it again:

The organs in the the torso of the dead body decay and fill with gas from the decay, and therefore, It becomes lighter and floats. The head and skull already has brain, so the gas can't enter it. And therefore, they stay heavy and sink.

Hope it helps, again :)