A shoal is a shallow area in a body of water, often created by a sandbar, rocks, or coral. Sailors must navigate carefully around shoals because their ships can run aground and get stuck if the water isn't deep enough. On old nautical maps, cartographers marked dangerous shoals with warnings so captains could steer clear of them.

The word connects to “shallow,” and you can see the similarity: both describe places where the water isn't deep. When a river becomes shallower, it shoals. Areas where shoals form often attract fish because the shallow water warms quickly in sunlight and provides good feeding grounds.

Confusingly, shoal has a completely different meaning when talking about fish: a large group of fish swimming together. A shoal is a loose gathering, while a school of fish moves in a more coordinated formation. You might see a shoal of herring or a shoal of minnows.