The earliest known sailing instructions for voyagers along the coasts of Arabia are to be found in a work known as The Periplus of the Erythraeah Sea. The author is unknown, but from internal evidence it can be dated to approximately 120 A.D. It describes the sea route from the Gulf of Suez to a town known as Barygaza in India near the mouth of the Narbada River in the Gulf of Cambay.
My point about the foregoing is the word "Erythraean." This is derived directly from the Greek word "erythros," meaning "red." It is clear from the text of the Periplus that, to the Greek navigators, the Red Sea was not just the gulf from Aden to Suez but also what we now call the Arabian Sea, including the Gulf of Oman. Sailors in those days seldom left the sight of land and even less often continued their journey by night even though the Pole Star was familiar to them. Hence their voyages were virtually always in coastal waters.
Now for the observation. One day in the summer of 1967 when I lived in Dubai, I took my family to the beach at Al Hira which lies between Sharjah and Ajman. When we got near the beach we were assailed by a most pervasive smell which made us think some sea creatures must have been washed up and were decomposing. However, when we reached the sea we noticed a large number of red patches on the surface which were coming ashore by wind and tide. As the patches dried out on the beach they gradually lost color until they were just areas of white scum. The fishy smell came from the decomposing matter. I understand that this phenomenon is caused by an algae, Trichodesmium erythraeum, which from time to time proliferates and produces the strikingly brilliant red patches on the surface of the sea. Hence a possible derivation of the Erythraean, or Red, Sea.
It would be interesting to know if any other members of the Group have observed this phenomenon. Should it be noticed, please report it immediately. Recording the locality and frequency of occurrence would help further research.