## 5 Conclusions  
Blockchain can be regarded as the future of the Internet, which has the potential to change the way the current economic and social systems work. The unique characteristics of blockchain help in asset protection, the adoption of cryptocurrency, smart contracts, voting, and so on. This proves to be advantageous in the IoUT ecosystem as well. Even though IoUT communication systems have proven to be beneficial for various systems, ranging from small observatories to harbors to oceans, they pose several challenges. This paper introduced the background related to the IoUT and blockchain and explained how such an integration would help address the challenges in the current IoUT ecosystem. An exhaustive survey was then carried out on the various blockchain-enabled IoUT applications, and we pointed out the benefits of each. Finally, the open issues and challenges in blockchain-IoUT integration were critically analyzed, and the possible solutions were presented.  
Conceptualization, S.B., N.V. and R.C.; methodology, R.C., N.V. and M.R.; software, M.R., G.C.S.; validation, S.D. and P.K.R.M.; formal analysis, P.K.D.; investigation, S.D., R.H.J. and T.R.G.; resources, T.R.G.; data curation, writing--review and editing, S.D., P.K.R.M., S.B. and N.V. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding  
**Institutional Review Board Statement:** Not applicable  
**Informed Consent Statement:** Not applicable  
**Data Availability Statement:** Not applicable  
**Conflicts of Interest:** The authors declare no conflict of interest.  
\begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Sl. No.** & **Challenge Type** & **Description** & **Possible Solutions** \\ \hline
1 & Computing Power & High computational power is required for the consensus algorithms to work & Lightweight mechanisms need to be introduced; devices with high computational power need to be used \\ \hline
2 & Storage Capacity & To handle a larger number of transactions per second and to store ledgers, high storage capacity is required & Use cloud resources \\ \hline
3 & Scalability & Scalable solutions are required to accommodate the huge number of transactions & Transactions to be carried out among subnetworks \\ \hline
4 & Standardization and Governance & To avoid the over-exploitation of resources and non-equitable service distribution, proper standardization is required & Proper standards are to be formulated; ocean governance needs to be considered regionally, nationally, and globally \\ \hline
5 & Migration from legacy systems & Migration from the traditional systems and technologies to adopt blockchain-enabled solutions is expensive and time-consuming & Rigorous training is essential \\ \hline
6 & Energy consumption & Acoustic communication demands more energy. Underwater communication suffers high attenuation, long propagation delays, and high bit rates; in addition, the blockchain adaption consumes high energy for successful transactions. & A lightweight energy-efficient blockchain-based framework for IoUT acoustic communication can be used to solve this \\ \hline
7 & Cost & Installing, managing, and maintaining the communication devices underwater is a challenging task. Frequent maintenance may be required such as battery change, sensor servicing, and so on & (1) Power-saving protocols can be used to reduce power use and also to avoid battery failures; (2) horizontal axis and vertical axis deployment strategies can be used to reduce the complexity of the network \\ \hline \hline \end{tabular}
\end{table}
Table 3: Challenges of the blockchain-enabled IoUT.