I note that boats on rivers and canals can vary in size, and are often quite small. Even ocean going ships could be very, very tiny a few centuries ago.
The Popham Colony in modern Maine built a 30 ton pinnace, the Virginia, in 1607-1608. The 46 colonists abandoned the colony in October 1608 and sailed back across the Atlantic in the terrifyingly small Virginia. Virginia made a round trip across the Atlantic in 1609 as part of the "Third Supply" fleet to Jamestown, and sailed to Virginia again in 1610.
The English ship Squirrel of only 10 tons made at least two transatlantic voyages in the 1570s and 1580s before disappearing on the 3rd voyage in 1583.
in Canada, the Red River Expedition of 1870 had to transport a thousand men, and supplies including cannons, hundreds of miles through a wilderness. Part of the trip was made in many canoes, which were portaged between various watersheds.
In the 17th century the English colonists in Maryland around the Chesapeake Bay and the Dutch colonists around the Delaware Bay carried on illegal trading and so couldn't sail by sea from bay to bay. So ships with their cargoes sailed up creeks from one bay as far as they could, and then were put into giant oxcarts and hauled for miles to the headwaters of creeks emptying into the other bay.
Some of those ships were only for coastal voyages but others were capable of transatlantic voyages.
The great city of Constantinople, "The City of the World's Desire", had a massive iron chain that was used to close off the Golden Horn from enemy ships during attack. A Russian chronicle claims that attacking Russian vikings in 907 hauled their small ships on land, attached wheels, and pulled them overland to enter the Golden Horn despite the chain, but Byzantine chronicles don't mention that attack.
And viking raiders and traders often carried boats and/or ships overland from one river system to another.
And the Ottomans did more ships overland into the Golden Horn during the seige of 1453.
And then there is the Diolkos the pathway, sort of an early railroad, to carry ships across the Isthmus of Corinth during Classical times.
Building canals to connect rivers to avoid portages is a good idea, but not totally necessary.
In 793 Charlemagne tried to dig a two kilometer long canal, the Fossa Carolina, from Truechtlingen to Weissenburg, connected the Schwabian Rezat river to the Altmuhl river, and thus the Rhine-Main and Danube watersheds. This may have been for commercial and/or military purposes. It is unknown whether this canal was completed and operational, but it was the inspiration for modern canals connecting the Rhine and Danube watersheds.
From what I've read, there were a number of vast earth moving projects in Europe in ancient and medieval times, including the Antoine Wall, Offa's Dyke, the Dannewerk, and the Great Fence of Bulgaria.
So building a Rhine-Main-Danube canal would not be totally out of scale in a medieval Europe.
With a medieval Rhine-Main-Danube canal one could sail and row and be towed from the Netherlands to the mouth of the Danube and sail across the Black Sea, and maybe travel up the rivers into Russia.
The Sea of Azov empties into the Black Sea, and the Don River empties into the Sea of Axov. The 101 kilometer long Volga-Don Canal connects the Don River with the mighty Volga River that empties into the Caspian Sea. The Volga River and its tributaries are connected by canals to the Baltic Sea and the White Sea.
I know that in modern Europe there are canals connecting the Rhone and Mediterranean with the Rhine, the Rhine with the Elbe, the Elbe with the Oder, the Oder with the Vistula, the Vistula with the Neman, and the Bug (tributary to the Vistula) with the Dneiper River that flows into the Black Sea. But some of those canals have locks and other modern technology that might be difficult to duplicate with medieval technology.
Another important modern European canal is the 98 kilometer long Kiel Canal across the Jutland Peninsula, allowing ships to sail from the North Sea to the Baltic Sea without having to sail around Jutland. A medieval version of the Kiel Canal could be used for civilian or military transport from the Atlantic Ocean and the North Sea to the Baltic sea.
So it would certainly be possible for a fictional state in medieval Europe to construct canals to make transportation in an east-west direction faster, easier, and cheaper for commercial and/or military reasons.
Canal construction is usually beneficial to society, but construction of the Grand Canal of China was considered a heavy burden by the perhaps short sighted peasants and was one cause for the rebellions that overthrew the Sui Dynasty.