## 7 Conclusions  
This paper presented a comprehensive review of the emerging role of 6G technologies in transforming MCNs. After highlighting the limitations of existing maritime communication systems, a reference architecture for heterogeneous MCNs was proposed, integrating terrestrial, aerial, satellite, sea-surface and underwater components to support seamless and resilient connectivity. The core features of 6G were then examined, including THz communications, RIS, ISAC, AI-native network design, and UAV-augmented coverage. It was also assessed how these innovations address key maritime challenges. Several representative maritime use cases were also presented. Furthermore, we proposed and mathematically formulated key optimization scenarios for spectrum allocation, energy-efficient UAV communications, and task offloading, reporting some potential solvers to these problems.  
A major enabler of future MCNs lies not only in technological innovation but also in regulatory evolution. Currently, national regulators have limited experience with mobile base stations deployed on ships, UAVs, or other maritime platforms, which raises significant uncertainty around licensing, interference management, and cross-border service continuity. The absence of dedicated and harmonized frequency bands for maritime mobile services hampers large-scale deployment of 6G technologies at sea, especially in international waters or port-adjacent zones where maritime and terrestrial cellular systems must coexist. To support truly global and resilient maritime communications, there is a pressing need for international coordination (e.g., via ITU-R or WRC), regulatory guidance on mobile infrastructure at sea, and the definition of flexible spectrum-sharing frameworks that balance sovereignty, performance, and safety. Addressing these regulatory gaps is essential for translating the architectural and optimization advances discussed in this work into real-world deployment.  
Future work may focus on implementing real-time and resource-constrained learning models in harsh maritime environments, such as reinforcement learning under partial observability or federated learning with intermittent connectivity. Integrating digital twins for predictive analytics, asset behavior modeling, and fleet simulation is another promising direction, particularly for use in remote coordination, maintenance planning, or port logistics. Validating the proposed architectural and optimization concepts in multi-agent emulation environments or through hardware-based testbeds is critical for practical adoption. Future research should also explore the integration of semantic and goal-oriented communication paradigms, especially in scenarios with ultra-low bandwidth (e.g., underwater links) or mission-critical requirements (e.g., autonomous vessel navigation). Additionally, emerging challenges such as joint orchestration of communication, computing, and storage resources, cross-layer AI decision-making, and resilient, privacy-preserving trust frameworks across distributed and multinational maritime systems offer important opportunities for multidisciplinary work. Overall, this paper outlines the technological trajectory toward a new generation of intelligent, adaptive, and autonomous MCNs and highlights the central role of 6G technologies in realizing this vision.  
Conceptualization, A.G.; methodology, A.G. and N.N.; validation, P.G., P.T. and T.S.; formal analysis, A.G.; investigation, A.G., A.  
**Acknowledgments:** This paper has been supported by the UNITY-6G project, funded by HORIZON-JU-SNS-2024, under grant number 101192650.  
**Conflicts of Interest:** The authors declare no conflicts of interest.