Information Sharing (IS) is essential for organizations to obtain information in a cost-effective way. If the existing information is not shared among the organizations that hold it, the alternative is to develop the necessary capabilities to acquire, store, process and manage it, which will lead to duplicated costs, especially unwanted if governmental organizations are concerned. The European Commission has elected IS among public administrations as a priority, has launched several IS initiatives, such as the EUCISE2020 project within the roadmap for developing the maritime Common Information Sharing Environment (CISE), and has defined the levels of interoperability essential for IS, which entail Semantic Interoperability (SI). An open question is how can IS performance be managed? Specifically, how can IS as-is, and to-be states and targets be defined, and how can organizations progress be monitored and controlled? In this paper, we propose 11 indicators for assessing SI that contribute to answering these questions. They have been demonstrated and evaluated with the data collected through a questionnaire, based on the CISE information model proposed during the CoopP project, which was answered by five public authorities that require maritime surveillance information and are committed to share information with each other.