When you walk around Reimeiden, you will see this roofed shelf behind the building. These structures are often found in temples near main buildings in inconspicuous locations. This type of shelf is known as an akadana in Japanese and provides space to change the water for cut flowers that decorate the altar within the main building. At Remeiden, the water can be taken from a well that sits just around the corner.
The aka in akadana is spelled “閼伽” with Chinese characters. These are characters that were chosen for the word based on sound alone, the characters do not express the meaning of the word. Aka refers to water that is given as an offering, it could be translated as “holy water” in English. The popular belief is that this aka and “aqua” from Latin have the same etymology. There are beverages in Japan that start with “aqua” so while Latin may not be widely studied here, there is an understanding of “aqua” being related to liquid/ water.
However, there is research that suggests that this popular belief is just folklore and that the two words are actually unrelated. How the belief that aka came from “aqua” came about I am not sure, but the simplicity of the explanation must be one reason it has stuck around even in the face of research to the contrary.
Ninna-ji Official English Blog 