## 1 Introduction  
The oceans cover over 70% of the Earth's surface; however, communication underwater is still a challenging task. Traditional communication systems based on acoustic waves, radio frequency (RF), and magnetic induction suffer from issues such as signal attenuation, interference, and bandwidth limitations, hindering their reliability in aquatic environments [1, 2, 3]. These limitations become more critical when trying to attempt high data rate transmissions for real-time communication in underwater scenarios like search and rescue, research explorations, and military operations. Visible Light Communication (VLC) provides an effective way to address these limitations by using light for underwater communications. With high bandwidth, immunity to electromagnetic interference, and minimal absorption and scattering, VLC attracts more interest in the research community as an alternative to underwater communications [1]. On the other hand, VLC has the potential to overcome environmental restrictions faced by traditional underwater communication systems, especially with respect to applications that require high-speed data transmission, such as underwater exploration, environmental monitoring, and autonomous underwater vehicles (AUVs) [4; 5]. However, several factors influence the effectiveness of VLC in underwater scenarios; the attenuation and scattering of light in water especially have an adverse impact on communication performance. These challenges are exacerbated by a number of factors, such as the turbidity of water, the distance, and the alignment between the transmitter and receiver. Studies indicate that underwater communication systems based on VLC may face issues such as the limited range of light propagation, especially in murky or turbulent water, and the need for precise alignment between transceivers to maintain a reliable communication link [6]. The difficulty of ensuring alignment and overcoming adverse aquatic channel conditions remains a significant barrier to the widespread adoption of VLC for underwater communication. Many new methods have been advanced to help solve these issues, for instance, the proposal of an adaptive VLC-based underwater monitoring system with a parallel multihop structure to extend its communication range and improve its signal strength [4]. In addition, other innovations involve the use of end-to-end learning techniques, which proved quite useful in automating signal processing, with a view to enhance the robustness of the communication system against environmental disturbances like optical turbulence [7]. Moreover, efforts to develop models for path loss in different water environments have led to more accurate predictions of VLC performance, helping optimize system design for varying underwater conditions [6; 8]. These advancements demonstrate the potential of VLC systems in meeting the growing demand for reliable and high-speed communication in underwater applications. Apart from that, the peculiarities exerted by the underwater environment itself, such as the impingement of sea waves and the dynamical movement of transceivers, have to be taken into consideration. Studies in the vertical VLC links, when one of the transceivers is situated at a buoy on the ocean surface, showed the need for developing theoretical models that can simulate path loss and random fading owing to periodic distance changes in transmission [9]. This review aims to study the integration of the latest technologies and approaches into a functional underwater communication system. The main contributions of this paper are an in-depth analysis of the principles, challenges, and future prospects of underwater VLC, with its promise to go beyond the limitations of traditional communication methods such as acoustic and RF communication. Various modulation schemes, including adaptive and hybrid modulation schemes, are discussed in the paper for optimizing data transmission in underwater VLC systems.