I think you are making a premature conclusion. Depending on who you ask, a living human body is 65 to 98% water. Dry the body to remove the water and seal it sufficiently so it does not soak again quickly and it will float nicely, like a piece of wood (or a duck).

Even if we assume that it is the air that makes the body float, you are making the wrong assumption that the air needs to be in the lungs. The air can be distributed uniformly throughout the body as a natural by-product of decomposition - something I have been told the zombies are particularly partial to.

One of my favourite Python moments! And I loved the other clip YT showed me immediately afterwards. The "Scientific Creation" museum. Pity I have no business in Kentucky in the foreseeable future.