You breathe in oxygen via your lungs.
The heart pumps blood to the lungs, where it (the blood) picks up oxygen & returns to the heart.
The heart then pumps this (oxygen-rich) blood round the body, supplying all the vital organs and tissues with oxygen.
Then, once this oxygen has been delivered to the vital organs and tissues, it returns to the heart.
The heart then pumps this (oxygen-depleted) blood back to the lungs, where it (the blood) picks up more oxygen and returns to the heart. And so the cycle is repeated.
That's what happens in a healthy person.
However, if the heart is not pumping at maximum efficiency, the blood flow can get sluggish when it's pumped towards the lungs.
This sluggishness of the blood flow can result in some fluid leaking out from the blood stream into the lungs, thus preventing the normal transfer of oxygen to the blood.
This condition (excess fluid leaking out in the lungs) is known as 'pulmonary oedema.' People with severe pulmonary oedema thus have too much fluid in the lungs, which prevents the normal transfer of oxygen to the blood stream (i.e. similar to drowning due to the lungs being full of fluid).
Seems like this poor chap died from pulmonary oedema.
Hope this makes sense, and I hope he rests in peace.