[spetznaz]

Those things are the most dangerous creatures prowling the seas today. A sting from one (and in a worst case scenario several) of them on a swimmer will lead to what is described as the most intense pain imaginable (together with intense backache, headache, shooting pains in their muscles, chest and abdomen, nausea, anxiety, restlessness, vomiting, and potentially pulmonary oedema and complete heart failure), and when a person is out at sea that basically implies drowning. The Irukandji (the name of this particular type of jellyfish) is worse than any shark (living or extinct) that ever called the sea home.

Now, sharks are obviously the most known of the aquatic dangers, but to be honest with you the Irukandji is a far greater danger. When it comes to jellyfish most people know better (at least I hope they do) than to mess with them, and thus they will stay away from the Portuguese Man-o-war (which I had float maybe 10 feet from me once), and the larger variety of Box jellyfish .....however the irukandji (which is related to the Box) is only mere centimeters long. For all practical purposes it cannot be easily seen, and woe unto anyone who swims into a mass of them. Now that is one bad way to go.

Now, this article doesn't say whether it was irukandji that got inside the Reagan, but the photograph shown is of the Irukandji.

[CarrotAndStick]

You're right. They cause extremely nasty conditions.

[TalBlack]

In terms of pure lethality I've heard the Rock Fish and that tiny blue octopus described the same way.