Pentland Firth go monstrous currents, and there are places where violence grows worse; one must pass by the Merry Men of Mey, and between the Swalchie and the Wells of Swona and around the bores of Duncansbay. (See also here.) Before daring these, the merman's children found a lee on the Caithness coast..." (The Merman's Children, Book Four, Chapter II, p. 200)
Off the island of Stroma, which is separated from the mainland by a strait three miles broad, is a small vortex called the Swalchie
-copied from here.
Another occasional feature of Poul Anderson's texts is extremely condensed information, as here. The map image shows both Pentland Firth and Caithness. In fact, this read like yet another Epic Journey, the difference in this case being that I can drive from Lancaster to Caithness although not to most of Anderson's exotic locations.