This kind of problem won't be solve until encounter data is checked for previous gens, which is easier said than done since it requires an exhaustive database not only of all in-game events (starters, fossils, gifts, in-game trades... for all versions of the game since RSE) but also a database of all past events. Furthermore, this would be still prone to errors because the encounter location and level are both lost during transfer to gen 5 which is the main reason encounters are only checked for gen5.
The Old Sea Map was never distributed outside Japan.