## 3 Applications
### Localization  
Positioning of underwater devices and other data sources is a key requirement for any location-based underwater application. As the parameters of terrestrial and underwater environments differ in various aspects, the systems adopted for the localization of terrestrial objects may not scale well to fit the needs of underwater systems. Extensive research has been carried out in the field of the localization of underwater objects. A detailed review on various localization schemes for the IoUT were discussed by the authors in [80; 81; 82]. The stratification effect and the mobility characteristics make the localization of underwater sensor nodes an even more challenging task to achieve. Apart from these, security and privacy are the key challenges associated with the localization of underwater nodes, especially due to the harsh environmental conditions. As most of the localization schemes are designed for suitable environments, there is the possibility of privacy violations, which may lead to serious consequences, especially in mission-critical applications. The authors in [83] explained the privacy and security concerns with respect to localization in underwater environments. However, very minimal research activities have been actively carried out for dealing with the privacy concerns in the localization of underwater sensor nodes.  
The blockchain can be regarded as an effective solution for dealing with the privacy aspects related to localization in underwater environments due to its unique characteristics such as decentralization, consensus, and security. Third party participation can be completely removed if blockchain technology can be integrated with the IoUT. This enables data to be stored securely without requiring any intermediate authority. The data generatedfrom IoUT devices can be processed and validated with the help of blockchain mechanisms. The data privacy issues pertaining to the positioning of underwater devices can be solved efficiently with the help of blockchain technology. The decentralization, consensus, and security aspects of blockchain primarily ensure that the privacy of data is not questioned. This enables location-based applications to adopt blockchain in the existing system. The flip side of applying blockchain in underwater sensor networks is its never-ending demand for power, memory, and computation [84]. The blockchain can be successfully integrated with the IoUT only if the underwater sensor nodes are capable of providing the above said aspects at a low cost. However, many researchers have channelized their efforts in this regard to enforce privacy in the IoUT with the help of machine-learning- [85], deep-learning-, and reinforcement-learning-based systems.  
A \"multilevel sensor monitoring\" architecture was proposed by [73], wherein a layer-based system was implemented for storing and processing data from the IoUT using blockchain technology. A lightweight consensus mechanism was also adopted in this work to process various transactions in an efficient manner. Additionally, the lightweight protocols help in transferring the data to the concerned system. Such an architecture can be efficiently implemented for the localization of underwater devices due to its lightweight protocols and consensus mechanism. \"BCTrust\" is a blockchain-based authentication mechanism proposed by [86] that is robust, secure, energy efficient, and transparent. This mechanism can be ideally used for systems with storage, energy, and computational constraints. This system can be extended to work with the localization aspects in the IoUT, as it can provide privacy and deals with the harsh underwater environments. The integration of blockchain with the IoUT significantly helps in the localization of various entities associated with the IoUT in a privacy-preserving manner. This is an open research area where the adoption of lightweight technologies has great potential to enhance the current blockchain-based IoUT systems.