Caption: A nautical chart showing the tides (i.e., the intertidal zone) of the East Friesland coast---the Wadden Sea (Dutch: Waddenzee, German: Wattenmeer West Frisian: Waadsee)---from the late 19th century or early 20th century.

This is Map B in The Riddle of the Sands (1903), a classic nautical spy thriller by Erskine Childers (1870-1922). Childers was English, but joined the Irish in Irish War of Independence (1919--1921). He was later executed by the Irish Provisional Government in the Irish Civil War (1922--1923) for the illegal possession of a small handgun given to him by Michael Collins (1890--1922).

The Riddle of the Sands (1903) is one of those great old stories where men were men and women stayed as the romantic interest and didn't try to take over the plot.

There seems to be only one film version: The Riddle of the Sands (1979 film, 99 minutes) which starred Michael York (1942--) and Jenny Agutter (1952--) (see also IMDB: The Riddle of the Sands (1979 film, 99 minutes)). See The Riddle of the Sands (1979 film): theme | 4:42.

But there is the TV series Das Raetsel der Sandbank, 10 episodes: 8 hours: See also IMDB: Das Raetsel der Sandbank (1987 TV series).

The "sands" of The Riddle (1903) are the Wadden Sea coast. The Wadden Sea is an intertidal zone (and also a tidal flat) and is the gray shaded area on Map B. The tides cover and uncover the Wadden Sea making it an exotic and dangerous physical environment, but rich in intertidal ecology.

Yours truly does NOT recall the term Wadden Sea being used in The Riddle of the Sands (1903) and it's NOT on Map B.

Selected locations: including bodies of water, cities, countries geographical features, islands, landmarks, political divisions, towns, tourist sights, et cetera:

Aurich, Aurich (district), Baltrum, border (AKA frontier), Borkum, Bremen, Delfzijl, Den Helder (off the map to the west in the Netherlands), Dollart Bay, East Friesland, Emden, Ems (river), Ems-Jade Canal, Esens, Lower Saxony, Frisia (AKA Friesland), German Empire, Germany, Groningen, Hage, Harle Seegat, Holland (AKA Netherlands), Jade (river), Jade Bight (AKA Jade Bay), Jever, Juist, Langeoog, Lower Saxony, Memmert, Netherlands (AKA Holland), Norden, Lower Saxony, Norderney, North Sea, Rottum, Spiekeroog, Wadden Sea (Dutch: Waddenzee, German: Wattenmeer West Frisian: Waadsee) (which is a intertidal zone and which is the gray shaded area on Map B and is the "sands" of The Riddle of the Sands (1903)), Wangerooge, Weser (river), West Friesland, Wiesmoor (surrounded by a raised bog which is also a moor (AKA fen)), Wilhelmshaven.

Credit/Permission: Anonymous map maker of the firm Walker & Cockerell (see also Emery Walker (1851--1933) and Sydney Cockerll (1867--1962)), before or circa 1903 / Public domain.
Download site: Wolfram Fassbender's The Riddle of the Sands site (now a dead link alas). Circa 2022, all maps in The Riddle of the Sands (1903) are at Cine-Tourist: Maps in Books.
Image link: Itself.
Local file: local link: riddle_of_sands_map_b.html.
File: Maps file: riddle_of_sands_map_b.html.