This paper presents some of the challenges related to multi-robot cooperation for the marine environment. Special attention is given to the world representation topic and to the communication challenges. Ontologies represent the tool to store and dynamically update world information. Due to the conditions of the underwater domain, communication among robots presents several issues. The exchange of information between the local world model of each robot, and those of the other robots needs to properly address specific points, such as limited bandwidth, reliability of the acoustic channel, selection of the information to be shared with other vehicles and information merging with previous knowledge of the world. Three scenarios will be then analysed: the Pandora project, with an emphasis on persistent autonomy, world modeling and failure management through appropriate ontologies; the Trident project, which deals with joint missions with an Autonomous Surface Vessel (ASV) cooperating with an Autonomous Underwater Vehicle (AUV), and the Arrows project, which envisages the use of a fleet of AUVs for underwater archaeology operations.