A dying man does not float when the black water takes him. When the panic stops and he loses his mind to the cold, his mouth opens wide. The lungs fill with heavy water. The air leave. The body becomes much heavier than the sea. He sinks to the dark bottom fast. He rests in the mud. He stays there in the cold.
The floating comes much later. It takes days.
Deep inside the dead stomach, the small bacteria wake up. They eat the flesh and make gas. The gas has nowhere to go. It swells the dead man like a tight balloon. The gas is lighter than the water.
Only then does the corpse break from the mud. It rises to the sun.
The dead float. The unconscious sink.