Coastal and underwater cultural heritage is increasingly exposed to climate-driven stressors, including sea-level rise, shoreline erosion, salinization, extreme weather events, and marine environmental change. These pressures are accelerating the degradation of vulnerable archaeological sites and historic coastal assets, while also exposing the limits of conventional documentation and conservation approaches based on periodic surveys and fragmented datasets. In this context, Digital Twins (DTs) are emerging as a promising paradigm for heritage monitoring and preservation, as they can integrate digital representation, sensor-based observation, environmental information, and predictive analysis within a dynamic and potentially interoperable system. However, current applications in cultural heritage remain uneven and frequently technology-centred, with limited attention to institutional embedding, environmental coupling, and long-term knowledge continuity. To address this gap, this study proposes a multidimensional maturity assessment framework for Digital Twins in coastal and underwater cultural heritage. The research adopts a sequential mixed-method design. First, a bibliometric analysis of 346 publications published between 2010 and 2025 was conducted using Scopus and Bibliometrix in RStudio to identify thematic trends, research fragmentation, and underrepresented dimensions in the literature. The results show a rapidly growing field, but still organized around weakly connected clusters, especially between digital representation, environmental modelling, and governance- or knowledge-related themes. Second, these findings were translated into a five-dimensional maturity framework comprising Technical, Structural, Environmental, Governmental, and Social dimensions. The proposed framework redefines DT maturity as a multidimensional preservation condition rather than a purely technological stage. It enables the assessment of whether a DT can function not only as a digital model, but also as an interoperable, governable, and sustainable preservation system. The findings indicate that current initiatives are advancing more strongly in technical and structural terms, while environmental, governmental, and social dimensions remain significantly underdeveloped. The framework, therefore, offers an evidence-based tool for identifying implementation gaps, supporting strategic planning, and guiding the more balanced development of Digital Twins for long-term heritage preservation.