Maritime transportation plays a critical role for the U.S. and global economies, and has evolved into a complex system that involves a plethora of supply chain stakeholders spread around the globe. The inherent complexity brings huge security challenges including cargo loss and high burdens in cargo inspection against illicit activities and potential terrorist attacks. The emerging blockchain technology provides a promising tool to build a unified maritime cargo tracking system critical for cargo security. However, most existing efforts focus on transportation data itself, while ignoring how to bind the physical cargo movements and information managed by the system consistently. This can severely undermine the effectiveness of securing cargo transportation. To fulfill this gap, we propose a binding scheme leveraging a novel digital identity management mechanism. The digital identity management mechanism maps the best practice in the physical world to the cyber world and can be seamlessly integrated with a blockchain-based cargo management system.