The fronds of giant kelp are kept at or near the sea surface via gas-filled floats.
Photo showing drift deposits of kelp (giant seaweeds) : large floats belong to the algal species Nereocystis luetkeana, tiny floats, blades and entangled cords belong to Macrocystis pyrifera. Taken in San Diego
"Gas-filled floats keep [kelp] fronds at or near the sea surface." (Koehl 1977:1067)
Journal article Mechanical adaptations of a giant kelpKoehl, M. A. R. ; Wainwright, S. A.