## 3 Underwater Routing Protocols
### Mees  
The mobile energy-efficient square (MEES) routing protocol uses two mobile sinks at the longest possible distance from each other [36]. Their movements follow predefined linear paths to collect data from the nodes. The protocol improves throughput, energy consumption and network lifetime. However, rather than moving to the locations where nodes have data ready for transmission, the sinks follow predefined paths that cause packet loss and delay. In addition, nodes drop packets when the sinks are not within their communication range. Furthermore, the network performance is poor in sparse conditions.  
Lotus  
The range-based low-overhead localization technique (LOTUS) relaxes the condition of using four sensor nodes as reference nodes to accomplish the localization of nodes [37]. It uses two reference nodes to localize nodes. This relaxation is helpful in identifying the position information of nodes in sparse conditions. However, with two reference nodes, the probability of error in position estimation of the nodes is higher than in four nodes.  
GDflood  
The geographical duplicate-reduction flooding (GDflood)-based routing protocol [38] considers the knowledge of the localization information of sensor nodes and combines it with network coding to overcome high energy consumption and duplicate packet transmissions. Energy consumption of the network is reduced by taking into account only suitable forwarders based on their position information. The network coding combines multiple packets into one or more single packets to reduce duplicate packets. However, the end-to-end delay is high due to the use of acknowledgment messages during data forwarding among the nodes.  
Dtmr  
The authors in [39] argue that signaling overhead in UWSNs is costly as the sensor nodes have limited battery power. Furthermore, this adds to delay in the communications. Therefore, to reduce the energy consumption and delay, a source node sends the data directly to the destination node or through a mobile relay node. This is called direction transmission or mobile relay (DTMR) communications. The choice of sending data directly to the destination or through the mobile relay is depicted as a trade-off between the delay and the throughput of the network. However, direct transmission to the destination may not guarantee reliable delivery of data packets as the underwater medium is highly unpredictable and harsh.  
Fvbf  
The authors in [40] argue that in selecting forwarder nodes in VBF, the position information of the sensor nodes alone is not enough to select the best forwarders. This is because the underwater environment is unpredictable and harsh and nodes change their positions and drain their battery power. As an improvement to VBF, a fuzzy logic-based vector-based forwarding (FVBF) protocol is proposed. The forwarder nodes are selected based on the valid distance, projection and the level of the battery power of the sensor nodes. The valid distance determines the extent to which a node is close to the sink node. The projection of a node determines its closeness to the routing pipe. The protocol achieves efficiency in energy consumption, throughput and end-to-end delay. However, nodes in the selected cylindrical vector die rapidly due to the heavy load of data, as they are frequently selected for data forwarding.