A narrow piece of land separating two water bodies is called an isthmus.

Here are some examples:

* Isthmus of Panama: Connects North and South America and separates the Caribbean Sea from the Pacific Ocean.

* Isthmus of Suez: Connects Africa and Asia and separates the Mediterranean Sea from the Red Sea.

* Isthmus of Corinth: Connects mainland Greece to the Peloponnese peninsula and separates the Gulf of Corinth from the Saronic Gulf.