Which feature is mandatory on all boats operating on U.S. waters?

A working horn or whistle is mandatory on all boats operating on U.S. waters because it serves a critical role in ensuring safety and communication while boating. These sound signaling devices are essential for alerting other vessels of your boat's presence, particularly in situations where visibility is limited, such as fog or during nighttime operations. They help prevent collisions by allowing boaters to communicate their movements and intentions effectively on the waterway.

The requirement for sound signaling devices is codified in federal regulations and is aimed at enhancing safety for all waterway users. Having a functioning horn or whistle allows boaters to comply with the "rules of the road" that govern navigation, contributing to overall maritime safety.

Other options may pertain to specific situations or types of boating activities, but the necessity of a sound signaling device is universally applicable to all boats, reinforcing its importance in maintaining safe navigation practices.