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Wrong. The Hungarian Kingdom had coastline only between 1779 and 1921 (Fiume). Saint Stephens Crown and the Kingdom of Hungary was two different thing. The Kingdom of Hungary was only one from among the possessions of the Holy Crown. The Croatian kingdom and the Dalmatian citystates had a coastline before 1779.
Anyway, the solution for your problem is a Lower Danubian Seaway, like the Great Lakes St. Lawrence Seaway:
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