From what I have read, there are a lot of variables involved in the changing buoyancy of a human body put into a body of water.
Muscle and bone tissue is dense and will not float. Fat and organ tissue is generally buoyant and will float. In theory, a lean, muscular person will sink while an overweight person will float. Air trapped in the lungs postmortem will cause many bodies to float for many hours until water seeps into the lungs. A drowning victim, however, will tend to die with considerable water in the lungs.
A body that sinks will become more buoyant as decomposition releases gas inside the body and causes it to expand. Punctures to the body, pre or postmortem will allow gas to escape. As a body decays, breeches in the body cavity will occur so gas will escape. A body that sinks may float to the surface as gas causes the body to expand. This will occur if the was body expands enough due to gas before decomposition, damage to the body and the action of aquatic organisms cause gas to leak from the body. Two critical factors that determine whether or not this will happen is how lean and muscular the body was and water temperature. Generally, colder water will slow internal decomposition more than it will *advertiser censored* damage to the body so a body in colder water is less likely to eventually float to the surface. Also, salt waster is denser than fresh water so, all other factors being equal, a body is more likely to eventually float in salt water.
The upshot of this is that a lean, muscular persons body in a cold fresh body of water may never float to the surface while a less lean, less muscular persons body in a warmer salt body of water, will probably float to the surface. An overweight persons body may never sink until decomposition causes the tissues to separate. How this all pertains to Joshs case is uncertain.
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