"He saw the phosphorescence of the Gulf weed in the water"
Artist's rendering of various types of plankton
Gulf weed, or Sargassum
Phosphorescence is the phenomenon whereby ocean water glows in the dark when disturbed. It's actually caused by bioluminescent plankton.
Gulf weed is another name for Sargassum (referred to later in the novel as "Sargasso weed"), a common brown seaweed.