Sargassum natans
Sargassum natans is a species of Sargassum. In English the species goes by the common names common gulfweed, narrowleaf gulfweed, or spiny gulfweed. It occurs in the Sargasso Sea. It is also pelagic and reproduces by fragmentation. It along with Sargassum fluitans have been credited with plaguing the Yucatan beach tourism industry. and South Florida. References 

Fucales