The reason people have to "anchor" bodies down to is to keep the gas filled object from surfacing too soon. It is in the best interest of the person hiding a body on a water bed to keep it down there as long as possible so that it will do ALL decomposing on the bottom and NEVER float to the top.
In this case Laci's body broke away from the anchors...the reason she surfaced without a head, hands and lower legs...the body cavity was full of gas and the constant pull against the anchored items allowed the gas filled cavity to break away...once she surfaced the atmospheric pressure was released from her body (33 feet = 1 ATM at the surface it would be 0 *if you are a scuba diver you will understand HOW the weight of water has an effect on the human body and blah blah) once her body surfaced...it didn't have the WEIGHT of the water on her and it allowed the baby to be expelled from her torso.
Therefore the lungs can no longer be ONLY looked at as being "occupied" by a substance may it be water or a gas to assume a live birth...at that point in time the whole body had been invaded by water and gas.