A narrow piece of land sticking out into the sea?

It can be called a peninsula, a cape, a headland, a promontory, or a point depending on its shape and the nationality of the person who named it.

Generally, though not always, a peninsula is a large body of land attached to the mainland via an isthmus.

A cape, from the Latin for 'head' can be any shape or height as long as it sticks out from the coast.