Final answer:
An inconspicuous, partly submerged vessel being towed should display a single white light at the aft end, and if the vessel is longer than 50 meters, it should display two diamond shapes in a vertical line.
Step-by-step explanation:
When an inconspicuous, partly submerged vessel is being towed, it must display certain lights and shapes to indicate its presence and ensure the safety of other vessels. According to the International Regulations for Preventing Collisions at Sea (COLREGS), a vessel being towed should display a single white light at the aft end. Additionally, if the vessel is longer than 50 meters, it must display two diamond shapes in a vertical line to signify it is being towed. These lights and shapes warn other vessels of the towed vessel's presence and help prevent collisions.