Box jellyfish is actually highly, highly venemous. Per UC Irvine Department of Emergency Medicine:
"They are even deadlier than cobras (Brinkman, 2015). Box jellyfish sting symptoms are venom-dose dependent and range from mild local pain to death. Their sting can also cause Irukandji syndrome, which causes a complex sequela of symptoms that could manifest as hypertension with later hypotension, trembling and extreme pain. Pulmonary edema or intracranial hemorrhage are also possible"
joycebell