The diagnosis of drowning can be difficult as autopsy findings are in most cases not specific and require further forensic testing.
The signs of drowning depend on the delay in recovering the body and on the breakdown of the body and tissues which alter the obvious indicators of drowning. One of these would be large amounts of froth present around nostrils and mouth and also present in the upper and lower airways. But that is only an obvious finding in a body that has just drowned and has not been in the water for hours or even days.
One would think – don’t they just have to check the stomach and lungs for water? It’s not that easy. In drowning, the inhaled volume of water can range, from quite small to very large. It has been shown that even a small amount of water, particularly cold water, may induce vaso vagal reflex or cardiac arrest reflex.
It induces WHAT you ask? Well, a sudden and intense stimulation of nerve endings in the skin, the mucosa of the eardrum, the pharynx, or the larynx by cold water (i.e. Lake Michigan this time of year for example only) can lead to a cardiac reflex arrest. It’s assumed that 10% of the drowned humans die after laryngospasm or breath-holding without actually aspirating fluid. In most of the rest of cases, when water is inhaled and passes through the alveolar-capillary interface and enters the circulation, the phenomenon of destruction of surfactant and of the alveoli architecture leads to asphyxia. During the entering of water into the bloodstream, the diatoms (unicellular algae belonging to the class of bacillariophycae living in fresh, brackish, or seawater) may reach the internal organs. Post autopsy, if the results are inconclusive, further testing for these diatoms in the organs may be done. Under strict extraction and identification conditions, these particles are good markers of drowning. In addition to a diatom test they may also do a histological and toxicological analysis (drugs/alcohol involved), blood strontium determination (another drowning marker), and biochemical analysis,
Sorry for the long science lesson but hope this helps explain a bit.