Few years back on a cruise we did a snorkel excursion in Martinique. They took us to a spot that was infested with these same purple box jellyfish. Wife got hit on shoulder and wrist. Her pain was excruciating. Tour people said they noticed them in the water days prior but never informed us prior to entering the water. Wife spent the night in the infirmary with an IV drip. Took weeks before skin healed.