Physalia physalis.
The man-of-war is actually a colonial jellyfish comprised of induvidual specialized polyps.
The man-of-war occurs primarliy in the tropical and subtropical regions of the Pacific and Indian oceans, and in the Gulf Stream. They are also in any warm seas throughout the world.
They eat mainly small fish using their tenticles (which can be up to 30 feet in length) to sting and paralyze their prey. They are eaten by animals such as the loggerhead turtle (Caretta caretta).
The jellyfish infamous for their very painful, powerful sting which can be fatal. The sting can cause shock, fever and cause interference with the lung and heart.

The Portuguese man-of-war, also known as the bluebottle, is not really a jellyfish, as commonly believed. It doesn't swim like a jellyfish - it floats on top of the sea, made buoyant by a gas-filled sac or "bottle" that gives it its alternate name. It is related to corals and sea anemones and is composed of four different types of polyps - float, tentacles, and digestive and reproductive polyps. The float is typically less than a foot long, but the tentacles extend many feet into the water. Part of the float is formed into a crest, which catches the wind as a sail and moves the animal through the water. Some Portuguese men-of-war are oriented to sail toward the right and others toward the left, which helps to disperse the species more widely throughout the ocean.

Common in tropical waters, the Portuguese man-of-war is often seen off the coasts of Hawaii, Australia, and the Florida Keys. Its sting is not usually fatal, but can be extremely painful. Some people have severe reactions to the sting and go into shock. Tentacles will often stick in the wound and need to be removed, which can easily cause further injury if not done carefully.