## 1 Introduction
The performance of these protocols is compromised when the network becomes sparse, i.e., when nodes are far apart from one another in the network or when the network has a significantly lesser number of nodes than the usually considered number of nodes in these protocols. With the dependence on geographical coordinates of nodes, the complexity of nodes' deployment is also increases, which further challenges the scalability of the network. For instance, the authors in [10] propose a routing algorithm in which two strategies are used. The first strategy involves connection of disjoint paths towards the sink so that nodes that are isolated from the rest of the nodes are actively involved in the routing process. This balances the load of the nodes and increases reliability. The second strategy uses the mechanism to avoid, bypass, or backtrack a void-hole. This further adds reliability to information advancement. However, the route construction involves geographical information of nodes, which increases the computational complexity and, therefore, reduces the scalability of the network. Moreover, information is sent over a single link, which may not be reliable all the time in the harsh sea channel. Even if these protocols are not dependent on the geographical coordinates of nodes [11; 12], advancing information over a single link in these algorithms threatens the reliability of information as the sea is a harsh medium. Packets are advanced in [11] with a unique holding time for every node in combination with increased transmission power of the node beyond which the void-hole exists. This provides reliability in reducing interference and avoiding the void-hole. However, increasing transmission power causes unbalanced energy consumption, which in effect, disturbs the stability period of the network. Moreover, for sparse network conditions, the holding time is not effective as nodes are not involved in frequent data advancement. In [12], the node facing a void-hole moves and adjusts its depth to avoid the void-hole. Then, it either selects a high energy relay node for information advancement to the end target or transfers directly to it. For sparse conditions, however, the mobility of the node for void-hole avoidance inserts extra propagation delay and causes position estimation error as depth information alone is not sufficient for void-hole avoidance. Moreover, high energy relay nodes close to the sea surface bear high data traffic and consume the battery power early. This further creates dead nodes close to the sea surface. As a result, this not only disturbs further information advancement to the sea surface, but also causes unbalanced energy consumption, which shortens the stability period of the network. Two address the aforementioned challenges, this paper proposes two routing algorithms for sparsely deployed WUSNs. They are: void-hole mitigation routing (ESRVR) and cooperative void-hole mitigation routing (Co-ESRVR). The ESRVR scheme uses a number of strategies to avoid the void-hole. Firstly, it checks that a data sender node has neighbors available up to two hops towards the sea surface. This overcomes the problem of the void-hole that is formed owing to blindly forwarding data packets without checking for the existence of neighbor nodes for further data forwarding. Since nodes are sparsely deployed, making nodes' connectivity beyond the two hops is unlikely to occur. Also, a single hop cannot detect a void-hole if it exists just beyond that single hop. Therefore, only two hop routing paths are considered for data forwarding. Secondly, in the scenario when a void-hole exists towards the sea surface, the node having data uses the backward transmission mechanism to search for other paths that are connected with the water surface. In the backward transmission mechanism, the sender node selects other nodes that are at the same or a higher depth than the sender node, but have connected paths using neighbor nodes up to two hops. This strategy continues unless information packets reach towards the sea surface.