Floating fingerprints

The whole of Sturla beach is currently covered in these little jellyfish. Has anyone got any idea what they're called exactly?

Actually, I say jellyfish but I only got that from a random man stood next to me on the beach and it's quite possible he was guessing. They don't really look like jellyfish and I couldn't actually see them in their natural form floating in the sea. They just look like thousands of blue fingerprints.