Even though most of the Earth's area is sheltered by the hydrosphere, the underwater world is nonetheless a challenging puzzle that has drawn the attention of scientists and researchers around the globe. Numerous theoretical and experimental research has been conducted to attain a safe and effective means of communication. This is crucial to facilitate the implementation of critical industrial, military and security applications and to guarantee the management of marine resources sustainably. Underwater communication technologies have come a long way as they have been implemented either utilizing wired and wireless techniques, through employing acoustic waves or electromagnetic waves (in the radio-frequency spectrum or the optical spectrum). Even though acoustic communication realizes long communication ranges in km, its limited data rate, its low speed, and its undesirable impact on the aquatic environment have all led to the realization of electromagnetic waves-based communication. Electromagnetic waves offer a high data rate and high-speed communication. Moreover, underwater radio frequency communications maintain a smooth transition between water and air, and they are unaffected by the turbidity properties of water. However, they come at a huge cost. Intriguingly, wireless communications in the optical spectrum have outperformed all the above techniques. This is owing to its high bandwidth and low cost. For that reason, we focus this review on different facets of the underwater optical wireless communications (UOWC) field, where we present an overview of the most important communication techniques and highlight their drawbacks and advantages. We also review the most basic physical processes that influence the light propagation and modulation underwater along with the coding techniques. We then discuss the link configurations and the system design, and we provide a summary of the most prominent studies from 1992 to the present.