## 2 Related Work  
This section describes routing protocols for UAWSNs that ensure reliability in packets delivery. The protocol in [5] considers reliable paths towards water surface in addition to interference by computing a cost function for every node. Computation of the cost function involves the physical distance between source and potential forwarder node, hop count of the potential forwarder from the sink and neighbor count of the forwarder node. The chosen forwarder node is the one with the smallest neighbor count and hops from water surface. The protocol achieves reliability in packets delivery to the sink node. However, it requires constant transmission of hello packets from the sink node that increases delay and energy consumption. In addition, the computation of the cost function involves too many parameters which compromises its computational efficiency. The protocol proposed in [11] ensures reliable delivery of data packets by following two strategies. The first strategy ensures that every sensor node has a unique packet holding time so that sensor nodes do not forward packets simultaneously. This overcomes interference and, in effect, increases reliability in data transfer to the water surface. The second strategy allows a sensor node to increase its transmission range in the situation when no neighbor exists. The increase in transmission range is limited unless the sensor node finds one or more neighbor nodes. This overcomes packets drop and, in turn, enhances reliability in packets delivery. However, the protocol has high energy consumption when a sensor node increases its transmission range as more power is consumed in this case. In addition, a node may not find one or more neighbor nodes despite of the increased transmission range when few nodes are present in the network. The grid-based adaptive routing protocol (GARP) [12] achieves reliable packets routing by dividing the entire network into grid cells. A node in one grid cell transmits packets to a node in another grid cell. When the connectivity of an existing path breaks, alternative paths are established for reliable packets transfer. However, the protocol performance is optimal only when the number of nodes in a grid cell is below a certain threshold. This limits the scalability and density of the network. Connecting disjoint paths to establish routes for reliable data transfer is accomplished in [13]. Nodes communicate using request and reply messages to update each other about the desired information. This information is combined with the forward ratio to establish routes for data transfer. The designed protocol is also environment friendly and protects the marine animals during data forwarding. However, updating information about disconnected routes is cumbersome when nodes are far apart and when nodes move with ocean currents. To cope with the adverse channel conditions, the protocol proposed in [14] combines cooperative routing with opportunistic routing. Cooperative routing ensures reliability by receiving more than one a single copy of identical data packets at destination. Opportunistic routing avoids burden on specific low depth nodes. However, the protocol does not check the connectivity of the paths over which the packets are forwarded. Consequently, reliability is not guaranteed beyond one-hop neighbors of the source nodes. The protocol proposed in [15] considers the underwater network as a cube and subdivides it into smaller cubes. Every cube has a cluster head selected based on the power it retains and its distance from the surface. Within every cube, selection of a relay node is accomplished on the bases of the existing power content of the battery, delay and distance from the water surface. The protocol ensures that the sink reliably receives packets. However, network is collapsed when the cluster head is dead. The authors in [16] argue that network coding and depth-based routing together improve the energy consumption, delay and reliability in packets transmission.