In the maritime field, integration of information service architectures and a common data model currently are topics of considerable interest. The International Hydrographic Organization (IHO) introduced the S-100 standard for the universal hydrographic data model and publishes Geographic Information (GI) registers in order to support the wider use of, the ease of access to, and the development of S-100 based product specifications. GI registers can be used with other domains' registers. Whereas each register might have contents for each domain or each specific type, extracting cross-domain or cross-type contents from registers is not easy. In this paper, we present three register ontologies based on the ISO 19135 register schema model, the S-100 model, and the FCD model, respectively. As each register ontology represents concept classes and their relations on different level, we also present ontology-mapping methods for enhanced interoperability of GI registers based on ISO 19100 series.