So apparently what Frisian sailors (and, subsequently(?), the English) called the “North Sea” is commonly called the “West Sea” (Vesterhavet) in Danish.
Makes sense, given that it’s mostly west of Denmark.
In that context, it’s amusing that the Icelandic Wikipedia article calls it Norðursjór “North Sea”, even though it should be something more like “South-East Sea”, if anything :). (For that matter, it should be the “East Sea” for the British….)
But to be fair, the Wikipedia lemma for Danish isn’t “West Sea”, either, but “North Sea” (Nordsøen), with “West Sea” mentioned as an alternate name—and the Icelandic Wikipedia article also mentions an alternate name, “English Sea” (Englandshaf), which is fair enough.
Edit: interestingly enough, the (west) Frisian (spoken in the Netherlands) Wikipedia article is Noardsee “North Sea”, while the North Frisian (spoken in north-western Germany and the islands off that shore) Wikipedia article is Weestsiie “West Sea”! (Though that's only in the Mooring dialect; the pages in other dialects in that wikipedia have “North Sea” as their name.)