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There are several species of Sargassum "weed" found floating on the surface off Bermuda. This is why the Sargasso Sea is named what it is.  If you go offshore, especially during calm weather, you can find huge rafts of this plant.  Mariners who were becalmed in the "Doldrums" or zones of low winds commonly found themselves surrounded by Sargassum.  These plants have gas-filled balls which you can see in the picture that keep the plant afloat.  The other common free living species is S. natans which has small barbs on the floats.  We have one species, S. bermudense, which lives attached to rocks near the shoreline. 

The rafts of Sargassum are home to an entire community of invertebrates and even a fish who has specialized to live camouflaged amongst its leaves.  Naturally, it is called the Sargassumfish (Histrio histrio) and it so closely resembles the plant that it is nearly invisible!