## 1 Introduction  
Underwater Wireless Optical Communication (UWOC) has received extensive attention from a large number of scientific researchers due to its many advantages such as high signal bandwidth, low transmission delay, strong confidentiality, and exemption from spectrum authorization [1]. In recent years, to meet the practical need of realizing the mutual transmission of observation information between airborne/space/ground nodes on land and underwater targets over long distances, cross-media hybrid relay transmission architectures of Radio-Frequency (RF)-UWOC [2] or Free Space Optical (FSO)-UWOC have emerged [3]. Reference [3] proposed a multi-hop Free Space Optical (FSO) and Underwater Wireless Optical Communication (UWOC) relay communication system that can be used for the tracking of Autonomous Underwater Vehicles (AUVs). In a fading environment, the expressions of the system's end-to-end average bit error rate (BER) and outage probability were analyzed and derived. For the FSO link, the influences of Gamma-Gamma (GG) turbulence fading and pointing error were considered, while for the UWOC link, the Lognormal distribution was adopted to model the effect of ocean turbulence. Both links used the Beer-Lambert exponential law to model the atmospheric and oceanic path losses. Reference [4] studied the outage probability, bit error rate, and channel capacity of a hybrid two-hop communication system composed of FSO-UWOC links under the Amplify-and-Forward (AF) mechanism. Among them, the GG atmospheric turbulence model was used for the FSO link, and the Exponential-Generalized Gamma distribution was used to model the underwater turbulence channel for the UWOC link. The impacts of atmospheric and underwater path losses on the system performance were not considered. Reference [5] studied the outage probability and bit error rate of a two-hop communication system composed of FSO-UWOC hybrid links under the assistance mechanism of Reconfigurable Intelligent Surface (RIS). For the FSO link, the GG atmospheric turbulence fading and pointing error were considered. For the UWOC link, only the Lognormal distribution was selected to simulate the ocean turbulence without considering the influence of pointing error. Also, the path loss factors were not considered for both links. It is obvious that all the above-mentioned studies modeled and analyzed the communication links between airborne/ground and sea surface relay nodes using FSO. However, in terms of RF-UWOC, which is the focus of this paper's research, Reference [6] analyzed the outage probability, bit error rate, and ergodic capacity of a RIS-assisted RF-UWOC hybrid two-hop system. Among them, the RF link was modeled as Nakagami-m and Rayleigh small-scale fading, and the UWOC link was modeled as EGG turbulence fading. The path losses encountered by the two links and the adverse effects of the pointing error on the UWOC link were not considered. Reference [7] derived the outage probability and bit error rate of a UAV-based RF-UWOC hybrid two-hop system in a fading environment and accordingly analyzed and solved the hovering height of the UAV that minimizes the system outage probability. Among them, the RF link was modeled as a hybrid model of Nakagami-m small-scale fading and atmospheric path loss, and the UWOC was modeled as only having EGG turbulence fading. Similarly, the effects of the pointing error and underwater path loss were not considered. Reference [8] studied the outage probability and ergodic capacity of a hybrid two-hop RF-UWOC system when using power-domain non-orthogonal multiple access technology to address two different underwater users and modeled and solved the optimal power allocation strategy for underwater users with the optimization goal of minimizing the system outage probability. Among them, the RF link was modeled as a small-scale Rayleigh fading channel, and the UWOC only considered the influence of the EGG turbulence fading factor. Reference [9] proposed an uplink fusion system of multi-hop UWOC and RF assisted by RIS carried by an AUV.