A drowning may not produce extensive findings. In fact, in 10 to 15% of cases, intense spasm of the larynx may even prevent water from entering the lungs.
The mechanism of drowning:
Stage 1: Surprise - lasts 5 to 10 seconds basically inactive, may inhale once or twice.
Stage 2: Respiratory arrest - lasting about one minute. Vigorous attempts to resurface. Mouth is shut.
Stage 3: Deep respiration - lasting about one minute. Take deep breaths and expel white foam to surface. Agitation ceases. Eyes and mouth open. May swallow a few times.
State 4: Respiratory arrest, 2nd. stage, lasting about a minute. Hypoxic convulsions may set in. Pupils are markedly dilated.
Stage 5: Terminal gasps, lasting for about 30 seconds (about three or four respiratory movements).
Death by drowning takes about four minutes in freshwater and about eight minutes in seawater. The less water inhaled, the greater the survival time.
In very cold water, where rapid cooling takes place resulting in hypothermia, the victim might survive longer.
A body will sink to the bottom of the water unless air is trapped in clothing. The body will float, head down, until bloating begins, at which time it will surface unless weighted down by something.
Factors used in determining the length of time a body has been submerged include:
Maceration of the skin
Cooling of the body
Rigor mortis
Livor mortis
Water temperature
Clothing
Decomposition
Adipocere formation
More graphic! details at:
What do drowned bodies look like at different stages of decomposition?