Blown in

Like  cast-off ballet pumps these flimsy husks are all that remains of a flotilla of by-the-wind sailors  now strewn among the seaweed debris. Each one was once a tiny inflated craft that skimmed, wind-driven,  across the surface of the water ingesting microscopic particles of food as it went.
They aren't, strictly speaking, jellyfish but fall into the same category of colonial entities that live at the mercy of the winds and waves. Their blue pigment protects them from UV light but they have no means to direct themselves and are no more substantial than  cellophane bubbles when they wash up on shore.

More about them here and here.

Comsiderably more substantial: seals in extra. They were lolling on the rocks far beneath the coast path, occasionally rousing themselves to slither and shuffle over the stones and seaweed. 
Now is the time they take a short break on land for the purposes of birth and conception.