For those of you who are ocean swim training please note that I have spotted a lot of Portuguese Man o War (called jellyfish but not actually of the jellyfish family) on South Shore this week so watch out for them and note that they have long tentacles below the water surface. They cause a very painful sting. See below from Wikipedia and how to treat stings.

This from Wikipedia :
Stings usually cause severe pain to humans, leaving whip-like, red welts on the skin that normally last two or three days after the initial sting, though the pain should subside after about an hour. However, the venom can travel to the lymph nodes and may cause, depending on the amount of venom, a more intense pain. A sting may lead to an allergic reaction. There can also be serious effects, including fever, shock, and interference with heart and lung function. Stings may also cause death, although this is extremely rare. Medical attention may be necessary, especially if pain persists or is intense, the reaction is extreme, the rash worsens, a feeling of overall illness develops, a red streak develops between swollen lymph nodes and the sting, or either area becomes red, warm, and tender.

avoid further contact with the Portuguese man o’ war and carefully remove remnants of the organism from the skin (taking care not to touch them directly with fingers or any other part of the skin to avoid secondary stinging)
apply salt water to the affected area (not fresh water, which tends to make the affected area worse
follow up with the application of hot water (45 °C/113 °F) to the affected area from 15 to 20 minutes which eases the pain of a sting by denaturing the toxins.
if eyes have been affected, irrigate with copious amounts of room-temperature tap water for at least 15 minutes, and if vision blurs or the eyes continue to tear, hurt, swell, or show light sensitivity after irrigating, or there is any concern, seek medical attention as soon as possible

Vinegar is not recommended for treating stings.Vinegar dousing increases toxin delivery and worsens symptoms of stings from the nematocysts of this species. Vinegar has also been confirmed to provoke hemorrhaging when used on the less severe stings of nematocysts of smaller species