The name of the sea may signify the seasonal blooms of the red-coloured cyanobacteria Trichodesmium erythraeum near the water's surface. Which gives the sea a red coloured look.
This sea is called the "Red Sea" because:
The Red Sea splits into two at its northern end into the Gulf of Suez, and the Gulf of Aqaba. The name originally applied to the Gulf of Aqaba, which...