Seas do not need to be saline in nature, they can be fresh water as the sea of Galilee. It's possible Sea III could be a a specially defined sea similar in nature to the Sargasso sea which has no coastline but is instead defined by the gyre. On the subject of seas, international law recognizes that all waters be they 'ocean' or 'sea' or 'gulf' are simply and plainly 'the sea.'
Generally what is considered a 'sea' need to be on the edge of an ocean and are semi enclosed bodies of water, but not as enclosed as a gulf or bay. Some seas are well defined areas such as the Sea of Japan, Mediterranean Sea, Black Sea, South China Sea, Caribbean Sea, or Bearing Sea. Others may be areas close to land that are defined by three coasts such as the Arabian Sea, Weddell Sea, North Sea, or Labrador Sea. While some are poorly defined such as the Amundsen Sea, Chilean Sea, Scotia Sea, Laptev Sea, East Siberian Sea, and Beaufort sea.
Oceans, rich in seawater, are the largest bodies of water. EX: Pacific, Atlantic, Indian, and Arctic. At the edges of the oceans are seas, a part of the ocean that is partially enclosed by land. EX: North Sea borders the Atlantic Ocean. Not every “sea” is actually a sea either. EX: Caspian Sea, Dead Sea, and Aral Sea lack an outlet to the ocean and are actually saltwater lakes. Gulfs and bays are water that jut into the land; the term gulf is usually given to the larger version with less enclosure, though these sometimes have a narrow mouth, and are (usually) almost completely surrounded by land. EX: Persian Gulf, and Gulf of Mexico.
The area north of 'C', the area south of 'A'/'B', the area between Cronaal and Nebula, are all areas that could be minor 'seas'.
Sea III could be called a gulf, but the fact it's not as closed gives itself more toward 'sea'.
I believe Hiskjriaana, Aurora Orb, Alta Italia, and Wolfsea among others have areas that could meet the definition of a minor gulf.