Which document is essential for the transportation of hazardous materials by vessel?

The essential document for the transportation of hazardous materials by vessel is the Shipper's Declaration of Dangerous Goods. This document is crucial because it provides detailed information on the hazardous materials being shipped, including their classification, packaging requirements, and emergency contacts. It ensures compliance with international regulations, such as those set forth by the International Maritime Organization (IMO) and other governing bodies, which mandate accurate and thorough disclosures about the dangerous goods being transported. This helps to ensure the safety of the crew, the vessel, and the environment during maritime transport.

The other options pertain to different aspects of hazardous material handling and transportation but do not specifically meet the requirements for maritime transport of dangerous goods. For instance, DD Form 2781 and DD Form 2890 are associated with different military or logistical transportation criteria, while DD Form 626 is typically used for inspecting hazardous materials during vehicular transport, not specifically for maritime cases. Therefore, the Shipper's Declaration of Dangerous Goods is the key document needed to facilitate compliance and safety when transporting hazardous materials by vessel.