## 2 Q1: What Are the Recent Trends of UIoT System?
### Prior Research  
Many articles discuss the latest research and applications developed in the UIoT system [4]. For example, in [5], [PERSON] et al. unveiled a survey on underwater communication technologies, including the pros and cons of using optical, acoustic and RF channels in the UIoT environment. Furthermore, the research shows that the RF channel is unsuitable for the underwater environment due to its high absorption rate. In [5; 6], the channel characteristics of electromagnetic (EM) signals in UIoT and the use of EM signals in the military application were discussed. In [7; 8; 9; 10], the challenges and merits of using acoustic signals in UIoT were discussed. Furthermore, the research shows that an acoustic signal reveals low absorption rates underwater. Therefore, the acoustic signal is used for long-distance communication in the UIoT environment, but the drawbacks are low bandwidth (1-100 kHz), limited speed (\(\approx\)1500 m/s) and high delay in data transmission.  
In [11; 12; 13; 14], the latest research on underwater optical communication (UwOC) techniques was discussed, and the strength and weaknesses of optical signals were shortened. Additionally, the research showed that UwOC are used for short-range communication with a high data rate in the UIoT environment, but UwOC cannot be applicable for long-range distances due to high attenuation.  
In [15], [PERSON] et al. developed a single hybrid optical, acoustic modem to achieve a high bandwidth rate, low battery consumption and long-distance data transmission. In [16], a built-in optical, acoustic communication technique was proposed by integrating the optical system into the existing acoustic communication technology to offer a high data rate, long-distance data transfer and low latency in underwater communication. In addition, from [17; 18; 19; 20], other acoustic-optical combined technologies were discussed. In [21], [PERSON] et al. proposed the new technique by considering multiple mediums and bandwidths based on the distance for reliable data transmission in the UIoT environment. In [22], [PERSON] et al. developed the underwater hybrid software-defined modem to support the fast and reliable communication system in UIoT. Figure 2 shows that the UIoT applications are grouped into five major categories and have numerous subdivisions according to the survey carried out by [PERSON] et al. [23]. Moreover, in [24; 25], the classifications and descriptions of each UIoT application are indicated.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Queries (Qs)** & **Discussion** \\ \hline
**Q1:** What are the current trends of the UIoT system? & UIoT is the growing trend in the current IoT system. Recently, numerous UIoT applications have been developed for the industries. Therefore, Q1 provides the survey based on the latest article and the recently developed UIoT applications. \\ \hline
**Q2:** What are the challenges of the UIoT system? & Challenge include technical challenges, security attacks and privacy issues. \\ \hline
**Q2:** What are the challenges of the current UIoT system? & Therefore, Q2 discusses the technical challenges based on UIoT channel characteristics and the possible security challenges and privacy issues in UIoT. \\ \hline
**Q3:** What are the possible methods to overcome the challenges, security attacks and privacy issues in the UIoT system? & In the UIoT system, most of the challenges and security issues are still of concern. Likewise, privacy methodologies are not yet considered for the current UIoT system. Therefore, Q3 highlights the countermeasures taken to overcome the challenges, security attacks and privacy issues of the current UIoT system. \\ \hline
**Q4 and Q5:** What are the findings and future directions? & Q4 discusses the findings based on the systematic review and Q5 highlights the future direction of this paper. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Research Goals.