By the wind sailor (Vellela)
Similar to Porpita, it has an additional asset, a transparent sail that enables it to move itself and the colony along the surface of the ocean. It also has small short tentacles and stinging cells under it's pale under surface.
The purple bubble snail (Janthina)
These belong to a totally different group - the Gastropod Molluscs, a carnivorous snail. Its muscular foot creates a raft of bubbles held together by special waterproof glue that enables Janthina to float beneath the surface film. It too is camouflaged with a blue upper surface and pale below.
When it bumps into a floating Cnidaria (e.g. Blue bottles, By the wind sailors and Blue suns), it will commence to devour the stinging tentacles of its prey with its rasp-like tongue.