Portuguese man o’ war is closely related to jellyfish.  When they are floating in the water they resemble an old warship at full sail. 

• Like jellyfish, Portuguese man o’ war can sting even when they are dead and have been on the shore for a while.
• Their sting is very painful for humans. 
• Treatment recommendations vary widely, one study suggests that treatment should be no different from jellyfish stings (vinegar and removal of stingers/tentacles).  Additionally, applying hot water or a hot pack for 45 minutes is recommended.

Sources: National Geographic & University of Hawaii at Manoa