Final answer:
The North Sea lies to the north of Charlemagne's empire, which encompassed a significant portion of Western Europe.
Step-by-step explanation:
The body of water that lies to the north of Charlemagne's empire is the North Sea. Charlemagne, also known as Carolus Magnus or Charles the Great, established a vast empire in Western Europe, which included regions from modern-day France to Germany and northern Italy. Notably, the Rhine- Main-Danube Canal, mentioned in the text, connects the North Sea to the Black Sea but serves as an inland waterway rather than a northern boundary.