## 1 Introduction  
The demand for faster and more reliable internet access is growing continuously, and the capacity of the existing radio frequency (RF) communication spectrum has nearly reached its limits. Hence, researchers are exploring complementary or alternative solutions to provide high-speed wireless communication. This exploration led to the utilization of visible light communication (VLC) [1, 2, 3, 4, 5, 6], which operates using a wavelength range from 380 nm to 750 nm as the carriers to carry the data. VLC belongs to the family of optical wireless communication (OWC) for providing promising and reliable wireless communication links in both outdoor and indoor environments [7, 8, 9]. VLC offers many advantages, such as providing an additional communication spectrum, immunity to electromagnetic interference (EMI), high transmission security and privacy, and no license requirement. Additionally, VLC can be cost and energy efficient by combining communication and illumination at the same time. One implementation of VLC, known as light fidelity (LiFi), not only provides communication and illumination but also supports movable users [10]. Furthermore, VLC is also regarded as one of the promising candidates for the future 6G mobile communication [11]. In addition to providing terrestrial transmissions, VLC can also provide underwater transmission [12, 13, 14]. Nowadays, a variety of underwater activities, such as underwater surveillance, marine monitoring, etc., are becoming crucial worldwide. Underwater sensors and autonomous underwater vehicles (AUVs) are widely adopted for underwater exploration. Although underwater communication can be realized using RF and acoustic waves [15; 16; 17], VLC could provide transmission merits. As shown in [18], visible light has the lowest absorption coefficient in water when compared with that in infrared (IR) and microwave. For example, visible light has about three orders of magnitude lower absorption coefficients in water than the 1.55 \(\upmu\)m IR light used in traditional fiber optics communication. Additionally, visible light has about four orders of magnitude lower absorption coefficients than microwave signals. Refs. [19; 20] provide good reviews for the recent progress and the state of the art of underwater VLC. Recently, 3.075 Gbit/s and 1.2 m underwater VLC transmission were achieved using a blue light-emitting diode (LED) and PIN photodiode (PD) [21]. Additionally, 14.81 Gbit/s and 1.2 m VLC transmission were reported using five-color RGBYC LED and PIN PD [22]. Apart from using LED, a laser diode (LD) can also be used for underwater VLC. A 12.62 Gbit/s and 35 m underwater VLC transmission were demonstrated using 450 nm LD and PIN PD [23]. Additionally, 30 Gbit/s and 12.5 m underwater VLC transmission were achieved using 488 nm LD and avalanche photodiode (APD) [24]. Furthermore, in order to significantly enhance the receiver sensitivity, underwater VLC utilizing a single-photon detector (SPD) [25] or a single-photon avalanche diode (SPAD) [26] was also demonstrated. Apart from underwater communication, underwater sensing and localization have also attracted much attention recently [27; 28]. One use of VLC or OWC is to employ a camera and image sensor to receive the optical signal, which is known as optical camera communication (OCC) [29; 30]. Nowadays, nearly all mobile phones are embedded with a complementary metal oxide semiconductor (CMOS) camera; hence, the OCC deployment cost could be low since there is no need to have significant hardware modification on the VLC receiver (Rx) side. As discussed in [30], OCC systems can be classified into region-of-interest (ROI) and rolling shutter (RS) operations. The ROI-based OCC systems usually use high frame rates and high-resolution cameras to receive the optical signal. Spatial multiplexing can be utilized to receive different light sources simultaneously to enhance the data rate.