## 1 Introduction  
UWSNs play a pivotal role in supporting a wide array of underwater operations--ranging from real-time critical tasks, like tsunami forecasting and military reconnaissance, to long-term environmental monitoring, marine biodiversity assessment, and subaqueous exploration [1, 2, 3, 4]. The effectiveness of these applications is deeply rooted in reliable channel estimation and precise synchronization mechanisms. In the absence of these, the sensor data relayed to the BS or courier node risks becoming fragmented or irrelevant, especially in latency-sensitive scenarios. Operating in complex, ever-changing aquatic conditions, UWSNs demand rigorous attention to data integrity and synchronization. Over-looking these foundational requirements can compromise mission success--particularly in contexts such as defense surveillance, where precision and timing are paramount [5, 6, 7, 8, 9]. The harsh nature of underwater environments, characterized by fluctuating physical conditions and high signal attenuation, has pushed researchers to develop robust MAC protocols that can endure these challenges. Over the years, the design of MAC protocols for UWSNs has become a vibrant area of study. This surge in interest is primarily fueled by the need to address underwater-specific issues, such as spatial-temporal uncertainty, multipath fading, energy limitations, and the difficulty of time synchronization [10, 11, 12, 13, 14, 15]. As a result, protocol designs have gradually evolved to reflect these constraints while ensuring energy efficiency, data reliability, and seamless communication across underwater networks [15]. Parallel to the advancements in UWSNs, UWANs have also witnessed growing academic and industrial interest. Their applications span navigation assistance, unmanned underwater exploration, maritime surveillance, and hazardous material detection, among others. As global interest in oceanic exploration intensifies, the significance of UWANs continues to rise. These networks typically consist of a mix of surface buoys, mobile nodes below the surface, and stationary underwater sensors (Figure 1). At the core of UWANs is underwater acoustic communication--currently the most feasible long-range, wireless communication method under water. This technology underpins collaborative underwater missions and supports diverse applications, including seabed mapping, marine ecosystem monitoring, ocean farming, and integrated sea-air-land systems [15, 16, 17]. However, UWANs are inherently constrained by slow propagation speeds, constantly shifting topologies, and the limited energy availability of their nodes--all of which can degrade network performance [18]. Some of the major hurdles in UWAN development include extended propagation delays, narrow bandwidths (often in the kHz range), and the limited computational and storage capabilities of underwater nodes [15]. Moreover, battery life remains a bottleneck due to energy-intensive operations and the impracticality of frequent recharging or replacement in remote underwater locations [15]. These challenges emphasize the need for highly specialized communication strategies tailored for underwater environments. Developing an efficient MAC protocol for UWANs is particularly challenging due to the unique physical properties of acoustic channels [14]. Compared to terrestrial networks, underwater channels suffer from greater delay, limited data capacity, and a higher likelihood of bit errors. Traditional MAC protocols--designed for terrestrial systems--tend to perform poorly under these conditions, often generating excessive control overhead and inefficient bandwidth use [19, 20, 21, 22]. While EM waves dominate terrestrial wireless communications, they are largely ineffective underwater due to rapid signal attenuation and seawater's corrosive effects, which compromise both transmission equipment and cables. Optical communication, though capable of high-speed data transfer, remains cost-prohibitive and limited in range.