A significant proportion of pollution of the marine environment is transmitted into the sea by transboundary rivers. The state of the marine environment increasingly depends upon the behaviour of states that do not belong to a particular maritime region. There is an obvious regulatory dichotomy between the environmental legal regimes dealing with marine pollution ('shoreline' regimes) and those governing international watercourses ('drainage/river-basin' regimes), which have historically evolved independently of each other. This creates problems of consistency and compatibility across different regimes, which have to be addressed in order to ensure the effectiveness of pollution-control measures throughout the entire pollutant transportation process. State practice has developed various practical ways of dealing with the issue of marine pollution from land-based activities in a transboundary context. The situation with river-borne pollution in the Danube River-Black Sea Basin provides an interesting case-study for critical examination as regards the practical aspects of the interface between such regimes.