## 1 Introduction
When the sink node is away from the transmission range of the sensor nodes, they send information to the minor sink nodes through cooperation with a single relay. In each zone, the node having lowest distance with respect to the sink node is considered as the destination node. The relay node has the second lowest distancefrom the sink node. The destination node receives the information from source and checks its bit error rate (BER), if it exceeds the desired threshold then sends a request (REQ) towards the relay node. The relay node amplifies and forwards (AF) the information bags and then sends to the desired destination node. The destination combines the received data bags using fixed ratio combining (FRC) technique. This process is continued for all regions. The sink nodes in the lower regions send their data bags to the sink nodes in the upper regions, that further forward the data bags to surface sink. To summarize, this paper has the following contributions:  
* For delay minimization in dense underwater WSNs, the DMR scheme is designed in which the whole network is divided into four balanced zones. Four sink nodes are placed at the center of all zones, one node at the center of each zone. Placing sink nodes at the center of each zone reduces latency during information broadcasting, as packets do not have to follow the complete journey towards sea surface. Unlike the existing schemes, the DMR scheme reduces the path length for information transmission to the desired sink node. Information packets take minimum interval of time to reach the targeted major sink node as compared to other schemes. The DMR also copes with the problem of data traffic on the desired sink node, which is caused by the information flooding over the upper major sink node. This further causes packets congestion and drop. In each region, the sink node secures the information of the nodes, which are then transmitted to the major sink node. This balances transmission of information packets and reduces the packet traffic on the major sink node. It also avoids the problem of packet collision. The information flow from sink nodes towards the top sink node in different intervals of time ensures the steady traffic of information packets. DMR offers lower delay and energy consumption in packets advancement than some prevailing protocols as supported by simulation. * Packets are sent in DMR over a single link, which may not be reliable always as sea channel is very fluctuating. Therefore, to improving information bags reliability in DMR, cooperative routing is introduced making it CoDMR protocol. In CoDMR, all nodes that cannot directly forward information bags to minor sinks due to limited range, use the destination and relay nodes. A destination node is at lowest physical distance regarding the sink and a relay node is at the second lowest physical distance with respect to the sink. The destination requests the relay for information transmission when the BER of information is above a specific threshold. The destination uses fixed combining technique to decide about the quality of the data and further transmission of information bags. CoDMR is more reliable than some prevailing protocols in packets reliability as supported by simulation results. * The distance calculation is based on timer operation and does not use geographical coordinates as are usually required. In addition, unlike the conventional approach, placement of minor sink nodes is independent of the geographical position of the sink nodes, as obtaining this knowledge is really challenging in sea environment due to ocean currents and limited resources. These strategies make the DMR and CoDMR schemes easy to operate and time efficient with less complexity as compared to geographical coordinates based protocols.