With the gradual shift of marine resource development towards large-scale and deep-sea exploration, traditional analytical and simulation techniques are no longer able to meet the growing demands of users. Digital twin (DT) technology offers a potential solution to address these challenges in the marine domain. This study focuses on DT technology for marine applications, providing a detailed discussion on the concept, application framework, key technologies, and future developments in the marine DT (MDT) field. A systematic review of relevant literature on MDT was conducted using the Web of Science database, analyzing the research focuses within this field. This study proposes a definition for MDT and presents a five-layer application framework, including the perception layer, data layer, model layer, fusion layer, and application layer. Key technologies for MDT are summarized, with a particular emphasis on data collection and transmission, data storage and management, modeling and simulation, and monitoring analysis and evaluation techniques, along with their applications in the marine domain. The future prospects of MDT are discussed, and the construction and application of a DT platform are demonstrated using marine engineering as an example. Furthermore, potential challenges in the development of MDT are analyzed, and possible solutions are proposed.