A more thorough explanation:

Narrow Sea

A narrow sea is a body of water that runs between two coastlines that are close to each other. The English Channel is an example of a narrow sea.

• The English Channel
• The Strait of Gibraltar
• The Sea of Marmara

These examples illustrate the definition of a narrow sea because they are all bodies of water that run between two coastlines that are relatively close to each other. The English Channel, for instance, separates England and France, which are only about 20 miles apart at their closest point. The other examples also have coastlines that are relatively close together.