It’s not just that Donau/Dunărea/Doana/Dōnaj/Dunaj/Duna/Donava/Dunav/Дунав/Дунай/Δούναβης/Danubio/Tuna/Danubi/Tunë is the second longest river in Europe, but it’s connected to Rhine and Western European river system via the Rhine-Main-Danube canal. You can actually go in inland rivers from France to Ukraine! Here is a huge pdf map of all Europe’s navigale inland waterways: https://unece.org/DAM/trans/main/sc3/AGN_map_2018.pdf