## 4 USWN Applications
In the 2D-USWN, the sensing nodes gather data and send them to the cluster head horizontally (first dimension), and the cluster head, which acts as a data hub, sends the data to the buoyant node vertically (second dimension) [21]. In the 2D-USWN architecture, the transfer of data between the cluster node and the buoyant node is preferably implemented using an acoustic signal due to its long-range capability and communication can be established across sensing nodes via RF, UWOC, and UWAC systems, depending on performance requirements [21]. In 2D-USWN architectures, applicable topologies can be mesh, star, or ring [21]. In 3D-USWN network architecture, the deployment of sensors is in the form of clusters anchored at varying depths [21]. This fact adds dimensions to inter-node communication [21]. [PERSON] et al. [21] reported that there are three communication scenarios for the 3D-USWN architecture: \"(i) inter-cluster communication of nodes at different depths, (ii) intra-cluster (sensor-anchor node) communication, and (iii) anchor-buoyant node communication\" [21]. In any of the three scenarios, either UWAC, RF or UWOC communication links can be deployed [21]. Finally, the 4D-USWN architecture is fundamentally like the 3D-UWSNs with the addition of a mobile USWN node such as an AUV, an ROV, a ship, or a submarine [21]. The mobile node collects data from various anchor nodes using RF or acoustic communication depending on the distance of separation between the anchor nodes and the mobile node [21]. Long distance of separation requires acoustic transmission, while shorter range RF can be launched [21]. [PERSON] et al. [21] expanded on the applications of UWSNs in different underwater domains, such as ocean monitoring, aquatic environment tracking, resources detection, deep sea surveillance, and mine detection and avoidance. With regards to monitoring, [PERSON] et al. [21] detailed specific projects where UWSNs are successfully implemented in water quality analysis, habitat and marine life monitoring, applications in fish farms, coral reefs, underwater exploration, natural resource detection, marine cable and pipeline detection and monitoring, disaster monitoring and prevention, and flood alert systems. In addition, [PERSON] et al. [21] also delved into military applications where cameras, imaging sonars, and metal detectors are integrated with AUVs to locate mines and to secure ports, submarines, and other assets of interest. [PERSON] et al. [21] concluded that despite the continued progress in USWN technologies, several challenges still remain unsolved, namely the unpredictable underwater environments, requirements for intricate USWN designs and challenging deployments and the difficulties in scaling existing solutions, power efficiency, and repair operations [21]; these observations are shared by [PERSON] et al. [60] and [PERSON] et al. [5]. Small-scale networks or single-hop solutions are still prevalent, and this scenario demands more innovative and scalable solutions to meet the implementation needs. Unreliable information due to continuous changes in water currents also poses a great challenge [21]. New protocol designs applicable to underwater media constitute an opportunity for new and innovative ideas [5, 21, 60]. Underwater communication seeks improvement in data rates, cost reduction, and devising solutions that prolong physical equipment lifetime [21]. Underwater conditions such as algal deposits on camera lenses and salt accumulation on hardware cause rapid degradation of USWN equipment [5, 21, 60]. Underwater media require dedicated protocols to ensure data transmission integrity and reduced BER. In other words, the underwater signal transmission still copes with the variability of the underwater channel and the physical properties of water. Although adaptation and prediction mechanisms have proven benefits and improved performance, they come with trade-offs, such as hardware power consumption, computation power requirements, and delay due to increased computation [5, 21, 60].