This is a Blue Button, a type of colonial hydrozoan related to the Portuguese Man-o-War. While it looks like a jellyfish, it is actually a colony of tiny creatures attached to the chitinous disc, which is hollow and filled with air to let the colony float. It does have stinging cells, which can be irritating, but are not dangerous to people. It is very similar to Velella velella, the by-the-wind sailor, but it does not have the small sail like the sailor.