## 2 Background
### Integration of Blockchain with the IoUT  
The principal motivations to integrate blockchain with the IoUT are discussed as follows:  
1. _Improving security in underwater communication_--The sensors capture the significant information and relay and send the data to the monitoring center present on the land surface. The information transmitted is highly sensitive, which brings several problems such as data stealing, network hacking, and breaking the communication systems [65]. There is also a dire need to provide secured and trusted solutions for the processing and storage of the enormous amount of data being generated from the IoUT devices. The blockchain-based network architectures provide solutions to the aforementioned challenges by providing immutability and trustworthy data sharing and management and also enable efficient monitoring and tracking of the underwater devices, processes, and related resources. Blockchain [66] ensures secured and trusted data sharing in the underwater communications without interventions from humans or third parties. Furthermore, the need for autonomous decision-making in the hostile underwater environment with fickle network connectivity with base stations is supported by blockchain. The smart contract features of blockchain enable such dynamic and autonomous decision-making, ensuring secured data storage and the reliability of such frameworks.
2. _Trustworthiness of IoUT smart devices_--With the substantial growth of IoT technology, more and more sensor devices are introduced, which are used for building the data communication network. Similar to the IoT, the IoUT also has several smart devices embedded with sensors, which are developed and utilized for data communication in the UWSN. The data generated from these UWSN devices may be very sensitive for critical applications such as defense [67]. The consensus algorithm is the decision-making process for the group of active nodes in the communication network while building trustworthy transactions in blockchain.
3. _Availability of the data in the IoUT_--Blockchain [68] uses the fault-tolerant mechanism, building an effective network for the availability of data transmitted through the IoUT devices. The decentralized and distributed structure of blockchain is a very important feature to confirm the availability of data perpetually. This property of blockchain is vital for making the data available to oceanographers to conduct their research analysis and investigations at their convenience [69]. The decentralized system storesthe information spread across the globe so that there is no single point of failure. This is achieved by storing the blockchain data over millions of devices on the distributed network of nodes; hence, the data and network are highly resistant to any malicious attack or technical failure in the network. Because of this, the availability of data in the blockchain-enabled IoUT communication network is possible.
4. _Privacy of the data in the IoUT_--Attackers cannot misuse or obtain the data, as the users can control their data with the private and public keys in a blockchain transaction, thereby enabling data ownership. The data owners can control when, how, and to what extent a third party can access the data. The privacy of data generated from IoUT devices can thus be preserved with blockchain technology.  
Furthermore, with the substantial magnification of underwater marine technology, the fusion of blockchain with the IoUT leads the way to explore the world's underlying water resources and can be best used for the benefit of humans and marine life. The motivation behind the integration of blockchain with the IoUT is depicted in Figure 4. In the following section, the notable applications of the IoUT are analyzed, discussing the role of the IoUT and its significance.