Here is a compilation on Man Of War jellyfish (I know they are a siphonophore, Not jellyfish, but no one will understand if I call them that), with fifteen facts you need to know. Be sure to read the link on avoiding getting stung!
Pixabay was nice enough to provide this picture of one on the beach. NOTE: They can still sting you on the sand, so, stay away!
I was surprised to see that some Man Of War tentacles are longer than a Blue Whale! Stings from a Man Of War have killed people, so if you see them, PLEASE be careful!
They are beautiful, but as with a lot of things in nature that are so brightly colored, they can be deadly! :D