where does one sea end and another begin
Borders of the oceans The major oceanic divisions are defined in part by the continents, various archipelagos, and other criteria. The principal divisions (in descending order of area) are the: Pacific Ocean, Atlantic Ocean, Indian Ocean, Southern (Antarctic) Ocean, and Arctic Ocean. Smaller regions of the oceans are called seas, gulfs, bays, straits, and other names.
North Sea The North Sea is bounded by the Orkney Islands and east coast of Great Britain to the west[1] and the northern and central European mainland to the east and south, including Norway, Denmark, Germany, the Netherlands, Belgium, and France.[2] In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean.[1][2] In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat,[2] narrow straits that separate Denmark from Norway and Sweden respectively.[1] In the north it is bordered by the Shetland Islands, and connects with the Norwegian Sea, which lies in the very north-eastern part of the Atlantic.[1][3]