## 1 Introduction  
Underwater WSNs has become an interesting discipline for research because of the exclusive applications. Such a network has sensor nodes usually deployed in an irregular fashion in the underwater environment. Main nodes known as sink nodes are positioned over the top of the ocean environment. The sensor nodes have the ability of reception and processing information and communication with the main nodes for variety of applications e.g., monitoring of the underwater environment, ocean sampling, assisted navigation, offshore exploration and tactical surveillance [1; 2]. Designing the routing protocols for underwater WSNs needs to overcome some challenges. The nodes under the ocean environment are constrained with the limited power of batteries, which are costly to be changed or replaced [3]. The acoustic channel has limited bandwidth. The acoustic signal propagates at low speed in the acoustic channel, which results in high propagation delay [4], as sea water affects radio waves in a severe manner. Minimizing end-to-end (E-2-E) latency is critical in underwater WSNs, as this parameter is involved in many applications (oil leakage, submarine tracking and detection) to send the encoded information with small delay to the desired destination [5; 6]. Latency becomes more critical in dense underwater WSNs where a large number of nodes have to interact with high data volume. If low latency paths are not chosen, the probability of loss of the high data volume increases, as data interacts with sea channel for long time. Several protocols have been proposed for delay minimization such as in [7]. However, delay is minimized at the expense of network reliability. Likewise, in [8], three routing algorithms for delay minimization are introduced by considering a weight function of lowest depth, holding time and the depth threshold. These algorithms achieve minimum delay but lose the throughput of the network. Another challenging task in the underwater WSNs is the data reliability. This becomes more important for dense networks where a large number of nodes interact. For securing the data reliability, cooperative communication provides one of the effective solutions [9]. The data reliability is crucial in military and data related applications [10]. However, because of the unavoidable characteristic of the sea channel, it is challenging to achieve a reliable network with small communication delay [11]. Reliability is further threatened as the density of the network enhances, as it causes data loss due to packets overhead. For packets reliability, the authors in [12] present a cooperative algorithm, where the fitness function is calculated in terms of lowest distance between the nodes and lowest depth information. However, it produces a high delay during packet forwarding. Another cooperative scheme is presented in [13], where the forwarder node is chosen on the basis of lowest depth. However, it consumes high energy due to the cooperation of two relays contributing in packet forwarding which also brings high latency. This paper designs two routing schemes for dense underwater WSNs. The first one is termed as the DMR protocol. For the sake of reducing latency, the network division in DMR is done in four equal segments and each segment has its own minor sink node to receive data from sensor nodes. This mechanism avoids reaching data to top ocean surface using multi-hopping, which shortens the latency, especially when the data volume is high with a large number of nodes in the network (dense network condition). Unlike the conventional approach, the placement of minor sink nodes in independent of the geographical knowledge of the position of nodes using a timer based operation. Data sent over a single link in DMR has reliability issue, especially if the link is worse affected by sea channel. Therefore, to add data reliability, cooperative routing is added to the DMR protocol that constitutes the CoDMR protocol. In CoDMR, the nodes closer to the minor sink node send information directly towards the sink node.