The Collaborative International Dictionary of English v.0.48:

gulfweed \gulf"weed`\ (g[u^]lf"w[=e]d`), n.
   1. A brown seaweed (Sargassum bacciferum) with rounded
      bladders forming dense floating masses in tropical
      Atlantic waters as in the Sargasso Sea.

   Syn: sargassum, sargasso, Sargassum bacciferum.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

gulfweed
    n 1: brown algae with rounded bladders forming dense floating
         masses in tropical Atlantic waters as in the Sargasso Sea
         [syn: gulfweed, sargassum, sargasso, Sargassum
         bacciferum]