## 3 Applications
### Environmental Monitoring  
Rapid urbanization and the development of industries have become an integral part of human growth. Due to this, water contamination and water deterioration have become alarmingly prevalent. Conventional water monitoring methods are lengthy, expensive, inefficient, and insecure, wherein manual testing is performed in laboratories. The monitoring of water quality involves the role of IoUT sensors in transmitting data as sound variations. The acoustic signals that are used for underwater communication consume high propagation delays and high error rates. In addition, sensors are deployed in an antagonistic environment, which remain unattended, making underwater communication more vulnerable to outside attacks. In order to secure sensors from being subjected to such  
Figure 4: Integration of blockchain in the IoUT. attacks, the authors in [73] proposed a blockchain-based multi-layer hierarchical architecture. This architecture simplified the monitoring and managing process of the IoUT data. Based on the residual energy cluster heads, sensor nodes were clustered and organized. The tracking of nodes and cluster heads was performed by a bloom filter, and two keys were used for communication. The standard secret key was used for the gateways to communicate, and a special secret key was enabled for cluster heads to communicate. Furthermore, the blockchain ledger helped store the routed data. The lightweight blockchain framework in [73] helped provide the legitimacy of data sources. The sensors deployed underwater for quality monitoring are prone to attacks, and there is the possibility for information leakage to occur. This is primarily due to the lack of security features in the underwater environment. A multi-layer hierarchical architecture proposed by the authors offers security to the IoUT nodes. The system significantly helps in protecting the sensors deployed underwater, securing the data transmission and organizing the networks in the underwater environment. The IoT is a category of pervasive systems that utilize embedded sensors, applications, and networks in smart systems and environments. The IoUT is one of the specific types of IoT systems that relies on underwater sensors in order to consume ocean information in the context of smart oceans. The oceanic information obtained from the group of sensors is very useful to carry out analyses such as water quality monitoring, marine ecosystem monitoring, water pollution monitoring, and so on. Hence, it is critical to create and provide a platform that allows the users to efficiently share and store the IoUT data in ad hoc unsecured environments. To provide distributed access to the users, the authors in [74] suggested the use of blockchain technology. The Ethereum smart contract mechanism can be used to share the IoUT data obtained from the oceanic environment. This smart contract mechanism helps in providing easier access to the users in order to store and manage access roles for the IoUT data. The suggested solution encrypts the IoUT data sent to the interplanetary file system and stores the hash values generated in the blockchain ledger. Sharing the valuable oceanic data over an unsecured ad hoc platform can question the trust and security aspects of the system. The Ethereum smart contract mechanism can be used in such cases to provide better security for sharing the IoUT data to perform various research analysis. A smart ocean requires various properties of the ocean to be explored exhaustively ensuring thorough understanding. The IoUT demands powerful technologies for creating a smart ocean. A detailed review of current advancements, future communication architectures, challenges, applications, and issues that need immediate attention were discussed by the authors in [75]. IoUT frameworks face similar security and standardization issues as in the case of the IoT. There exists no common standard for the regularization of communication and transmission between underwater devices.