What device rotates to power a boat forward or backward?

The device that rotates to power a boat forward or backward is the propeller. A propeller consists of blades that spin around a central hub, and as it rotates, it pushes water backward, which in turn propels the boat forward due to Newton's third law of motion—every action has an equal and opposite reaction. This mechanism is fundamental to how most motorized boats operate, allowing for both forward and reverse movement depending on the direction in which the propeller rotates.

The other options serve different purposes on a boat. A cleat is a fitting used to secure lines, allowing for docking and anchoring but has no role in propulsion. The gunwale refers to the upper edge of the side of a boat and is part of its structure and design. The bow is the front part of the boat and plays a role in navigation rather than propulsion. Understanding these definitions clarifies why the propeller is the correct answer in this context.