## 3 Applications
### Defense  
Naval defense exercise operations include surveillance, mine detection, sub-marine detection, and recovery operations. Underwater Autonomous Vehicles (UAVs) are programmable, self-propelled, and unmanned, capable of carrying out assigned tasks with little or no real-time input from humans [87]. Torpedoes are the first form of UAV used in naval defense systems [88]. Recent research and development witnessed marvelous growth in making these vehicles significant for naval defense applications. It is inevitable to extend the idea of the IoT for underwater things. The IoUT has significantly altered the way the data are collected and stored and the way intelligent decisions are derived from the same. The issues related to data privacy and security in navy-based defense activities are mostly because of the centralized architecture of the IoUT systems used for data storage. Here, data tampering may occur during data transmission between the nodes. Hence, the concept of the immutability and traceability of blockchain can address the aforementioned issues.  
Naval defense is one such sector that has enthusiastically adopted the advantages of the IoUT. The IoUT is an interconnected system of computing devices that helps to improve the productivity and efficiency of the data communicated among the water-based vehicles such as ships, submarines, drones, and other water vessels. The current IoUT ecosystem is expensive as it employs large servers to store the data in a centralized manner. Securing the data source is an extremely necessary requirement, since important decisions are obtained from the intelligent devices and stored in a common cloud. The data in the IoUT environment can be accessed and modified by the participating nodes, and the origin of the tampered data may be unknown. Additionally, the current IoUT control systems reduce the data transmission speed, leading to bureaucratic leadership among the nodes. There is also the associated risk of losing the data if there is no recovery mechanisms in place in case of system failures. Communication from remote areas between the IoUT and UAV facilitates vulnerability threats, wherein cyber criminals obtain unauthorized access to connected networks and devices, thereby exposing critical data. The distributed communication nature of the ocean network enables unauthorized users to access data and information by following unethical practices. The centralized architecture of the IoUT in such cases is insufficient in providing secured and efficient IoUT solutions. Blockchain is an advanced technology, which is used for securing and tracking real-time applications and their data. It is a promising technology that leverages the information sharing and accessing process. The authors in [89] emphasized the aspect of data exchange among the participating nodes in the Internet environment, ensuring the same to be secured and immutable. In the blockchain-enabled IoUT environment, the decentralized structure of blockchain is used, wherein all participating nodes share a copy of the transaction, and hence, transparency is maintained. Additionally, the cryptography features such as hashing, digital signatures, and public-private keys enable the data block to securely link with other blocks and, thus, ensures the immutability of the data. Another important aspect of underwater communications in defense is traceability. Each device or vehicle in the network is associated with an ID, which enables the user to track the transactions and generate smart contracts. Smart contracts are shared among various devices and sub-systems in the application. Each task in the system executes its operations using the smart contracts. The advantages of the blockchain-integrated IoUT is manifold in defense applications in oceans.