Hi IABL, I've found this Swedish map of the Baltic Sea during the Great War.
dark and light squares: Russian and British mined camps.
black points: wrecks.
black lines: British and Russian anti-submarine nets.
dotted lines: merchant routes.
dotted and underlined squares: British and Russian operation zones.
german cities: Danzig, Königsberg and Mamel.
Russian cities: from Libau all cities were Russian, but Libau was captured by german forces on may 7th 1915 and become a submarine base in early 1916 instead of Danzig and Riga was captured by German forces after the Battle of Jugla on september 3th 1917.
PS: sorry if the image is too big and sorry if this map is incomplete, but with some researches I discovered that in the Gulf of Finland there was a big mined camp and a submarine net that encircled like a first quarter of the moon the Fortress Island and naval base of Kronstadt.