Location of the Baltic Sea in the region.
The borders of the sea are based on those from the IHO's "Limits of Oceans and Seas" Draft 4th Edition, 2002, since these were closer to real-world usage than the 1953 3rd edition.
The map was created using data from Natural Earth and International Hydrographic Organization, IHO; Sieger, Rainer (2012): Limits of oceans and seas in digitized, machine readable form. Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, PANGAEA, with mapshaper.  (The former data source is in the public domain, the latter under CC BY.) For reproducibility purposes, this was the set of scripts used, but some slight post-generation editing was applied.

The style of the maps was generally intended to follow that of the German Locator maps project, except for the highlight colour of the sea, for which there was no apparent convention and for which dark blue (#4790c8) was used.
Own work
Aplaice