What is a quay?

A quay is a structure that is typically built parallel to the shoreline and is used as a place where ships can dock to load and unload cargo or passengers. This definition encompasses the functionality and design of a quay, which serves as a vital part of maritime infrastructure, facilitating trade and transportation.

In the context of the choices, the other options do not align with this specific definition: a small boat is unrelated to the concept of a structure for docking ships, a coastal area known for its tides does not specifically refer to a docking structure, and a landlocked harbor does not involve the concept of a quay since it implies a lack of direct access to the ocean. Thus, the correct answer effectively captures the essence of what a quay is in the maritime context.