This flag is for the Mediterranean Sea, or rather the flag for the 'Sea of Middle Earth.' It is broken into three parts that represent its three continental coastlines: Asia, Africa, and Europe.
Flag of the Mediterranean Sea
The yellow bar represents Europe's Coastline and is in the traditional position for north. The indigo 'bottom-southern' half represents Africa's Coastline. It is often a cultural bias that the western mind conceives this sea as a European Sea, but this is not the case. Asia and Africa are key cultural elements in making up the Mediterranean Sea.
Note that most if not all writing systems in Europe originate from Africa - via the Egyptian primary alphabet. Likewise most if not all mainstream religions of Europe originate from Asia - of the Abrahamic Traditions: Judaism, Christianity, and Islam.
Finally the coast line of Asia is represented by the fly green half with a white stripe denoting a separation yet connection to Europe and Africa. When you mix yellow and blue it makes the colour green. This reflects the fact that although Africa and Europe are separated they join hands through Asia. Only Asia touches Africa and Europe as the proverbial middle man.
The seahorse emblem was chosen since the Mediterranean Sea looks a bit like a sea horse already, with the outline of its head at the western end of the Mediterranean Sea.
It is the right of any nation that has a Mediterranean Sea watershed to fly this flag. It truly is the ancient Sea of Middle Earth since Africa, Europe, and Asia all meet at this water junction. Coincidentally the only opening to an ocean opens west - directly due west to Kitty Hawk, North Carolina in the Americas.
map image credit
Mediterranean Sea - Eric Gaba (Sting - fr:Sting) All rights released Own work ;
Raster background map : screenshot from NASA World Wind (Public Domain) June 2007