  Underwater drones have found a place in oceanography, oceanic research,
bathymetric surveys, military, surveillance, monitoring, undersea exploration,
mining, commercial diving, photography and several other activities. Drones
housed with several sensors and complex propulsion systems help oceanographic
scientists and undersea explorers to map the seabed, study waves, view dead
zones, analyze fish counts, predict tidal wave behaviors, aid in finding
shipwrecks, building windfarms, examine oil platforms located in deep seas and
inspect nuclear reactors in the ship vessels. While drones can be explicitly
programmed for specific missions, data security and privacy are crucial issues
of serious concern. Blockchain has emerged as a key enabling technology,
amongst other disruptive technological enablers, to address security, data
sharing, storage, process tracking, collaboration and resource management. This
study presents a comprehensive review on the utilization of Blockchain in
different underwater applications, discussing use cases and detailing benefits.
Potential challenges of underwater applications addressed by Blockchain have
been detailed. This work identifies knowledge gaps between theoretical research
and real-time Blockchain integration in realistic underwater drone
applications. The key limitations for effective integration of Blockchain in
real-time integration in UUD applications, along with directions for future
research have been presented.
