The Northern Sea is an ocean to the north or northeast of continent of Serenia (aka Daventry), and east of Serenia. It lies somewhat north-east of the Western Sea.
Background
The Northern Sea lies beyond the Great Mountains within the polar regions. Little is known what lies beyond the horizon there, mariners seldom stray from the sight of land in those rocky regions. The usual sailors' tales of great monsters and serpents large enough to swallow ships are told.
The seas contain magical waters (perhaps residue of Harlin's own powerful magic), that connected the game King's Questions to the fate of the ship taking Alexander and Cassima back to Daventry. The Green Isles and Mordack's Island are located in the Northern Sea as well. The area is also dotted by the Rocky Islets.[1]
The sea is dangerous due to the cold water and the Sea Monsters that live there. Other dangers include dangerous currents in parts of the sea, especially neat the green isles' Rocky Shoals. The area between Western Sea and norther see is also capable of great storms that can disrupt any compass, and send ships adrift towards the Green Isles.[2]
At one time, Harlin's castle was located in the North beyond the ocean (both the Western and Northern Seas), on a northern continent of his own devising in the the Unknown. His power was in its prime, and had used it to move the lands apart, creating the vast ocean which at the time divided Serenia into two landmasses (with the Endless Desert and Woods to the south, and the Great Mountains to the north). Only the Island lay in between.
Behind the scenes
In the first edition of the King's Quest Companion, there is no mention of the Northern Sea. The maps include in the book are loosely based on the maps seen in KQ3. On the main world map, the Western Sea encompasses both what would later be known as the Western Sea and the Northern Sea. The area near where Northern Sea and the Green Isles would be located in later editions is marked Here Abide Monsters. In fact Here Abide Monsters is a region marked on the map of Green Isles I'm third edition.
In the 2nd Edition the edges between both seas are a bit more fuzzy, with the Western Sea and Northern Sea kinda blending somewhere north of the continent. With the Western Sea marked on the northwest of the continent, and northern sea lying about northeast of the continent. By the third edition the Northern Sea is listed almost directly north of the continent, and Western Sea northwest of the continent