The stonefish has what is regarded as the most painful venom on earth.
The venom is injected through the 13 spikes along the back. When the stonefish senses danger, it raises the spikes, which are sheathed under its scales. When pressure is applied to their top, i.e. somebody stepping on them, the sheaths are pushed down, the spikes are exposed, and they begin to secrete venom.
Once in the blood, the venom cells are large and spikey. They travel around setting off pain receptors, and slaughtering red blood cells. The pain receptors cause... pain and the destruction of large amounts of red blood cells deprives the body of oxygen and leads to painful cramps.
Divers are said to drown in water, no matter what the depth, as after being poisoned they cannot swim.