Charles Darwin and earlier observers noticed "red water"

 March 18th. — We sailed from Bahia. A few days afterwards, when not far distant from the Abrolhos Islets, my attention was called to a reddish-brown appearance in the sea. The whole surface of the water, as it appeared under a weak lens, seemed as if covered by chopped bits of hay, with their ends jagged.
—Charles Darwin, The Voyage of the Beagle

 He credited a number of observers with earlier reports.  In his notes he mentions:

M. Lesson (Voyage de la Coquille, tom. i., p. 255) mentions red water off Lima, apparently produced by the same cause. Peron, the distinguished naturalist, in the Voyage aux Terres Australes, gives no less than twelve references to voyagers who have alluded to the discoloured waters of the sea (vol. ii. p. 239). To the references given by Peron may be added, Humboldt's Pers. Narr., vol. vi. p. 804; Flinder's Voyage, vol. i. p. 92; Labillardiere, vol. i. p. 287; Ulloa's Voyage; Voyage of the Astrolabe and of the Coquille; Captain King's Survey of Australia, etc.

In January 1915,  "Sea Sawdust" was reported by Earth Observatory:

Another image from Landsat 8, taken on 11 September 2017 is featured here:

From that site:

The blooms are likely to be Trichodesmium spp., a microscopic, photosynthetic cyanobacteria that aggregates into long strands on the sea surface. They are ubiquitous around the world, and they tend to bloom off the coast of Queensland between August and December as the water warms. Some evidence suggests that Trichodesmium blooms here are happening earlier and more often in recent years.

From a ship or the shoreline, these blooms look like dirty brown or green stripes on the water and like an oil slick when they hit the beach. Such blooms off the Australian coast were reported two centuries ago by Captain James Cook and by Charles Darwin.

Up Close and In Person 

It was not possible to easily link the following site, with photos of Trichodesmium sp.  

 See the University of New Hampshire Phycokey.