## 9 Future Trends and Research Directions  
The evolution of underwater communication systems is poised to enter a new era driven by emerging technologies and innovative approaches in research. Quantum communication, for example, promises to develop very secure and efficient methods of data transfer in VLC underwater systems. These could achieve unparalleled security and reliability by exploiting the principles of quantum mechanics, such as quantum entanglement and cryptographic key distribution. Quantum communication, while promising for secure data transfer, faces significant challenges in underwater VLC systems. The short operational range of optical signals in water, compounded by scattering and absorption, limits the practicality of quantum key distribution (QKD) and entanglement-based protocols. Recent experiments demonstrate that underwater quantum links are currently feasible only over very short distances (<10 m) in controlled conditions [84; 85]. Moreover, turbulence and particulate matter exacerbate decoherence, necessitating advanced error correction and single-photon detection technologies. While future advancements may overcome these barriers, current implementations remain largely theoretical or confined to lab-scale demonstrations. Another transformative development is the integration of RIS in VLC systems. RIS technology can dynamically manipulate the propagation environment for better signal strength, range, and coverage in challenging underwater settings. This capability makes RIS a pivotal enabler in overcoming the intrinsic limitations imposed by the nature of underwater light propagation, including scattering and absorption. AI will play a significant role in the optimization and management of hybrid VLC systems. Machine learning algorithms can analyze underwater conditions in real time to adaptively optimize modulation techniques, reduce latency, and improve the overall efficiency of communication networks. However, even with this advancement, there are some other challenges that exist at the standardization and commercialization level. Due to the non-availability of universal standards in VLC-based underwater communication systems, these may not be interoperable or scalable. The path ahead needs some collaboration among industry stakeholders, researchers, and regulatory bodies in providing robust standards that ensure seamless integration and wide-scale adoption. In the future, the long-term possibilities of VLC in water are endless. These can enable subsea research, exploration, and defense with high speed, low latency, and security for AUVs, sensor networks, and Internet of Underwater Things (IoUT) applications. As research and innovation continue to overcome the unique challenges of underwater environments, the vision of ubiquitous high-performance underwater communication systems is becoming increasingly attainable. While underwater VLC systems offer significant advantages in high-speed, low-latency communication for short-range applications (AUV docking, sensor networks, and real-time monitoring), their practical deployment is constrained by fundamental challenges such as limited range (typically \(<\) 100 m in clear water) and sensitivity to environmental factors like turbidity and turbulence [26]. Future research should prioritize hybrid systems (VLC-acoustic or VLC-RF) and adaptive technologies (RIS, AI-driven modulation) to extend operational range and reliability. The 'limitless' potential of VLC lies not in unbounded scalability but in its ability to address critical gaps in scenarios where traditional methods fail, such as secure, high-bandwidth communication in controlled or short-range underwater environments. Global Standards for Underwater Visible Light Communication (UVLC)Underwater visible light communication (UVLC) has gained significant attention due to its potential for high-speed, low-latency data transmission in aquatic environments. However, the lack of globally harmonized standards poses a challenge for widespread adoption.