What does the map show? (with my square frame marking):
Your selection shows most of the lake Żarnowiec and the German-Polish border as it was after 1920, when International Commission on Demarcation decided on the final border at the vicinity of the lake: the entire lake, together with 50 meters of the lake shore and the Nadole village on its west side has been allocated to Poland.
So I guess your riddle is about Nadole village, a half-enclave that had no direct connection with the rest of Poland, other than by boat across the lake or via so-called "neutral road" through German's Czymanowo, at the south end of the lake.
This curious solution was a result of Kashubian activity during the plebiscite; the village was 100% Kashubian and Roman Catholic.
Translated from pl.wikipedia.org/wiki/Nadole_(wojew%C3%B3dztwo_pomorskie)
At the end of World War I, when Poland regained its independence, as a result of the Parisian arrangements the border between Poland and Germany was drawn at the center of the Żarnowiec Lake. Nadole and half of the lake were to belong to Germany. Only after long and laborious efforts of local residents the village and the entire lake became part Poland. Nadolians are very proud of that fact since this kind of border corrections were the exceptional phenomena.