As you drown you no longer have access to oxygen, which within a couple of seconds causes injury to your brain and other organs. The exact process goes like this: Due to initial panic, you will automatically exert energy and use up more oxygen as you move around in the water.

The body’s response to drowning begins when the brain senses hypoxia (oxygen deprivation). This causes the nervous system to trigger a series of reactions meant to protect vital organs, e.g., the heart and lungs, from damage until the victim is removed from the water and resuscitation efforts begin.

Death can occur due to complications following an initial drowning. Inhaled fluid can act as an irritant inside the lungs. Even small quantities can cause the extrusion of liquid into the lungs (pulmonary edema) over the following hours; this reduces the ability to exchange the air and can lead to a person "drowning in their own body fluid".

Mere moments after expiring, body decomposition kicks in as bacterial enzymes start to break down the body's soft tissues and spread throughout the blood vessels. From there, it's a pretty predictable process of putrefaction, then bloat, purge, advanced decay, and finally, dry remains.