The Kronstadt Sea Gauge is a small object with an outsized role: the Baltic water-level reference from which elevations across Russia and several former Soviet republics have been measured. It sits by the water in a city built around naval engineering, fortifications, ice, floods, and the practical business of controlling the Gulf of Finland.
Do not expect spectacle. The interest is in the idea that a quiet mark on the waterfront helped define maps, railways, and engineering projects over a vast territory. It pairs well with Kronstadt's naval cathedral, docks, and flood-barrier story, especially for travelers who enjoy infrastructure with history attached.