The prolongation of land boundaries into the sea becomes very complicated where the boundary is defined by the mouths of large rivers with considerable change in coastal development. The cases of the boundaries Honduras-Nicaragua (Río Coco) and Nicaragua-Costa Rica (Río San Juan) are used to point out difficulties in applying definitions and concepts of the UN Convention on the Law of the Sea to this specific situation. The third case refers to the Nicaraguan Rise, where many cays, banks, reefs and islands represent serious problems of definition in the context of maritime delimitation. The persisting lack of detailed studies at the local scale, including historical change and coastal dynamics, is overruled by nationalistic territorial policies which intensify boundary disputes and conflict potential. This requires a combination of topographical, geomorphological and historical studies with conflict research which includes the political performance of the actors and its relation to ‘national’ interests, public opinion and consciousness of the maritime dimension of territory.