Both the Box Jellyfish and the Portuguese Man-of-War have long tentacles with stinging nematocysts, that deliver a powerful, painful sting, sometimes causing scarring, and in some very rare cases, cardiac arrest.

Most beaches will post warning signs, to alert you to the fact that there are jellyfish in the water or on the beach.

Follow local advice on beaches and take notice of warning signs before entering the water. 

First Aid Treatment for Box Jellyfish Stings

• Immediately flood the area with household vinegar to keep un-discharged nematocysts from firing. This does not relieve pain but prevents additional stings.
• Never rub the area with sand.
• Pluck off any vinegar-soaked tentacles with a stick or other tool.
• If the victim has shortness of breath, weakness, muscle cramps, palpitations or any other generalized symptoms, call for emergency medical help.
• For pain relief, apply ice packs. If pain becomes unbearable, seek medical advice. No studies support applying heat to these stings.
• Meat tenderizer, baking soda, papaya, commercial sprays, alcohol, and urine are not considered to be effective nematocyst remedies. They often have harmful effects, causing more damage.
• If a red streak develops between the sting site and neighboring lymph nodes, or if either area becomes red, warm and tender, seek medical help immediately. 

Do not delay in calling for emergency medical help if the victim shows signs of a significant reaction to the jellyfish sting.