## 4 Application of UWSN
### Monitoring  
The unplanned exploration and expiation of the ocean and seas have led to the destruction of the marine environment. As such, it has also opened up a new application of UWSN: environment monitoring. The monitoring of natural resources such as gas and oil, as well as natural habitats of animals and marine life, is becoming a very critical operation. With the help of UWSN, it is possible to detect changes in temperature, pressure, chemical properties, thermal properties, and most importantly, water quality. If the pollution can be detected at an early stage, researchers can take action to prevent collateral damage to the environment. In our study, we briefly discuss a few monitoring applications. 1. Water Quality Monitoring: Water is one of the most essential elements for all living beings. However, there is a shortage of water sources due to pollution. It is becoming a severe problem in most countries due to negligence in protecting water sources. Water becomes polluted when harmful substances are dumped into the rivers and sea without any filtration [74]. Hence, it is essential to monitor all water bodies' water quality Different techniques are used to measure the water quality of different water bodies such as oceans, seas, lakes, rivers, and pools. Ref. [75] discusses a few water quality monitoring systems using UWSN. 2. Natural Habitats Monitoring: Monitoring habitats is one of the most challenging aspects of UWSN application as it faces the adverse condition of the seas. At the same time, it is crucial to do so as the water supports life on and underneath it [19]. The monitoring is further sub-classified into three groups as follows: 1. Marine life: The main purpose of this is the surveillance of marine life for a giver of a targeted region under and above water. It is able to detect the movement of fish or humans within that certain area. There are many different protocols used to detect particular properties of water [76]. \begin{table}
\begin{tabular}{p{56.9 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Classification** & **Protocol Year** & **Key Idea** & **Advantage** & **Disadvantage** \\ \hline Direction-based & EBook (2018) & Used trust-based computation to optimize energy efficiency & Longer lifespan & High overhead \\  & SPIN (2002) & Uses validation to avoid regions & Void avoidance & Low packet delivery ratio \\  & EAVARP (2018) & Uses three phases to achieve load balance in the network & Load balanced network & Low delivery ratio \\  & OMR (2018) & Balances resources, and prevents bottleneck using multi-modal routing & Greater data transmission & Complex computation, and high delay \\ \hline Flooding-based & DD (2003) & Saves energy by using caching and processing data within the network & Energy efficient & No void detection \\  & RACAA (2019) & Uses special relay node to forward the data & Reliable path, and high data delivery ratio & High energy consumption \\  & L2-ABF (2012) & Uses angle calculation to forward the message & Does not need exact location & High energy consumption and overhead \\  & IHRP (2013) & Uses hybrid mechanism to reduce communication overheard & High delivery ratio & Needs exact location of the nodes \\  & iDFR (2017) & Adds QOS metric to DFR for better link stability & High delivery success rate, stable network link & High delay, and overhead \\ \hline \hline \end{tabular}
\end{table} 
Table 19: Comparison of direction aware routing protocol. * Fish Farm: The use of UWSN in fish farming is blossoming in recent years. Fish farming is very sensitive even to the slightest changes in their habitats and highly delicate. Thus, it requires constant monitoring. Different techniques and protocols are used to measure and keep the habitat in a healthy condition. Doing this will financially befit the farmers. In [77], the authors developed a two-mode underwater surveillance camera system consisting of a sonar imaging device and a stereo camera. It can estimate the quantity, length and weight of fish in a crowded fish school.