Caution: Portuguese Man o' Wars

Fishermen and sailors as well as visitors to the beaches have reported large numbers of the floating jellyfish-like organisms of Portuguese man o' war which can appear almost anywhere in the oceans where the water is warm enough. 

They appear to be particularly abundant currently on and off the south and west coasts of La Gomera, especially Valle Gran Rey. Swimmers should be aware of the danger and be vigilant. It may be better not to go for a swim at all until the floating invasion has disappeared again. The Portuguese man o' war can inflict a very painful 'burn' when merely touched and stings often require a visit to the doctor. A first aid remedy used traditionally by fishermen is fresh urine, believe it or not, but a visit to the doctor is advisable when badly stung. It is also important to avoid further contact with the Portuguese man o' war when carefully removing remnants of the organism from the skin: Take care not to touch them directly with fingers or any other part of the skin to avoid secondary stinging.

Wikipedia has this to say about them:

''The Portuguese man o' war (Physalia physalis), ..., often mistaken as a jellyfish, is a marine cnidarian of the family Physaliidae. Its venomous tentacles can deliver a painful sting.

Despite its outward appearance, the Portuguese man o' war is not a jellyfish but a siphonophore, which differs from jellyfish in that it is not actually a single multicellular organism but a colonial organism made up of many highly specialized minute individuals called zooids. These zooids are attached to one another and physiologically integrated to the extent that they are incapable of independent survival. ...

... Stings usually cause severe pain to humans, leaving whip-like, red welts on the skin that normally last 2 or 3 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, there is an extreme reaction, 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.''
UPDATE March 18th 2014:
The danger persists and some people were stung in the past few days. One person was so badly stung that an ambulance had to be called. 
UPDATE March 27th 2014:
Unfortunately the toxic organisms are still present in the waters of the south-west of La Gomera. Dead and stranded examples were seen again today. Do not handle these, as the toxin remains active for 24-48 hours after their death. 
UPDATE May 17th 2014:
The administration of Valle Gran Rey have re-issued their warning and have stated that coastal waters in the south-west continue to be infested with stinging/burning Portuguese Man o'Wars. Large numbers of the organisms have been sighted again and several swimmers were stung. The authorities are also appealing to the public to spread the word and caution tourists and visitors who may be unaware of the possible danger. Read more...