Middle English North-se, from Old English norþsæ, norðsæ, usually meaning "the Bristol Channel" (see north + sea). The application to the body of waternow so called, east of England (late 13c.) is from Dutch (Noordzee, Middle Dutch Noortzee); it lies to the north of Holland, where it was contrasted with the inland Zuider Zee, literally "Southern Sea"). To the Danes, it sometimes was Vesterhavet "West Sea." In English, this had been typically called the "German Sea" or "German Ocean," which follows the Roman name for it, Oceanus Germanicus. "German" persisted on some British maps at least into the 1830s. North Sea in Middle English also could mean "the northern portion of the ocean believed to surround the earth" (late 14c.).
the Greek thalassa erythra; the reason for the name is unknown; speculation has traced it to: 1. algae in coastal waters; 2. sandstone rock formations on the shores; 3. a tribal name; 4. ancient association of "red" with "south" (as "black" with "north").