Final answer:
A GPS receiver is not required on towing vessels over 12 meters in length within U.S. navigable waters; required elements include a magnetic compass, radar, and echo sounder.
Step-by-step explanation:
The piece of navigational safety equipment that is NOT required on towing vessels over 12 meters in length, provided that the vessel remains within the navigable waters of the U.S., is a GPS receiver. The key pieces of equipment typically required for maritime navigation include a magnetic compass, which is a fundamental navigational tool for determining direction; radar, which is essential for identifying and tracking other vessels and potential hazards, especially in poor visibility; and an echo sounder, used to measure the depth of water beneath a vessel to prevent grounding.
While a GPS receiver is a common and highly useful tool for navigation, providing precise location data, it is not considered a mandatory piece of equipment for towing vessels over 12 meters within U.S. navigable waters, as the other pieces of equipment can be used to ensure navigation safety and compliance with the maritime regulations.