Remember these? I learned of another possibility. The creatures might be colonies of porpita porpita, also called blue buttons. Apparently they float on the surface of the ocean.

"Porpita is found in all seas. It is pelagic, floats on or near the surface. Its size ranges from 2 to 10 cm in diameter. It is regarded as an inverted polyp-its body is discoid and contains a flattened disc-like, chitinous shell. The shell contains many air filled chambers and functions as a float or pneumatophore. Each chamber opens to outside by a couple of pores."

http://www.biology-today.com/general-zoology/invertebrate-zoology/polymorphism-in-porpita/