It's between Denmark and the United Kingdom (5,3)

I believe the answer is:

north sea

(Other definitions for north sea that I've seen before include "Marine area off Britain" , "UK's source of oil, gas" , "Part of the Atlantic between Britain and Scandinavia" , "A great deal of water" , "Waters east of the UK" .)

I've seen this clue in The Wall Street Journal.

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