###### Abstract
This recognition emphasizes the need for additional work within 3 GPP to address maritime-specific requirements, which should be carefully formulated with significant input from key stakeholders representing the global maritime sector, including maritime safety aspects. These requirements are poised to assume a pivotal role in the forthcoming 6G standardization, with a core objective of delivering evolved performance and tailored user experiences customized to meet the distinctive challenges of maritime environments. Consequently, there is substantial value in conducting a comprehensive review of standardized 5G enabling technologies, with a particular focus on integrating mobile RAN entities like satellite access components into 5G and beyond systems and supporting the concurrent utilization of multiple Radio Access Networks (RANs) including those mobile RAN entities. This effort takes into account the extended operational lifespans of the maritime communication technologies, which result in a diverse landscape of multiple access networks within the maritime sector. Effectively, harnessing multiple RANs simultaneously and efficiently is crucial for achieving ubiquitous connectivity, which is an essential foundation for the successful deployment of autonomous ships in the maritime sector. The key contributions of this paper are summarized as follows:  
* This paper contributes to the insightful analysis of Access Traffic Steering, Switching, and Splitting (ATSSS) features, which enable the simultaneous resource utilization of multiple RANs including mobile RANs within the 3 GPP standardization framework. This transport-layer solution offers substantial advantages to the maritime sector, preparing for impending shifts, including the adoption of autonomous ships that require ubiquitous connectivity during their voyages. * In addition, this paper makes a significant contribution by conducting a thorough examination of standardization trends and technologies within 3 GPP, with a specific focus on the integration of satellites and low-altitude Uncrewed Aerial Vehicles (UAVs) as mobile RAN entities. These mobile RAN entities, integrated into 6G systems, are poised to play a crucial role in ensuring uninterrupted connectivity and optimizing concurrent resource utilization across multiple RANs when combined with ATSSS features, given the anticipated increase in RAN heterogeneity in the near future as the maritime sector undergoes digitalization and mobilization, including the introduction of autonomous ships. * Finally, this paper contributes significantly by exploring opportunities and challenges for future research within the context of ATSSS features in the maritime communication scenarios in terms of three aspects encompassing the concept of autonomous ships as mobile RAN entities, the integration of legacy maritime communications into the 6G framework, and the variability in maritime channel measurements, generally employed as one of the criteria for selecting an appropriate RAN among multiple options, influenced by uncontrollable factors such as climate change. This paper is organized as follows: in Section 2, we present an overview of related work from academic research and global standardization trends, with a particular focus on ITU-R [13] and 3 GPP, for the provision of seamless connectivity over 6G systems for the evolution of the maritime sector including autonomous ships. Section 3 providesa comprehensive exploration of integrating satellite and low-altitude UAVs as mobile RAN entities into 5G and beyond. Following this, Section 4 explores transport-layer solutions for efficiently distributing data traffic across two access networks, catering to various applications within the 3 GPP standardization framework. In Section 5, we navigate the opportunities and challenges that warrant further investigation in the ever-evolving landscape of providing ubiquitous connectivity optimized for autonomous ships in the maritime sector. Finally, Section 6 offers our conclusions.