The link selection methodologies of three prominent underwater routing protocols are extensively been investigated through their network architecture and then performance has been analysed in regards of packet delivery ratio, end-to-end delay, network lifespan and network energy consumption. The findings are discussed as under.
A. Energy Balancing Routing Protocol for Underwater Sensor Networks (EnOR): Rodolfo W et al , discussed one of the immutability problems relating to the priority level of transmission of nodes., resulting in balanced power usage and extended lifespan of UWSN network. It rotates the priority level of transmission for the transmission nodes, taking into account the remaining capacity, reliability of the connection and progression of packets.
Link Selection methodology: A beacon packet is periodically transmitted by each underwater sensor node. The lightning packet comprises the identity of the sender, the remaining information on its energy and its size. Algorithm 1. provides the procedure for selecting the best connection.
Using i as a sensor node with a data packet to be sent while maintain the neighbouring table as Ni. The node analyses its adjacent nodes to choose the most suitable nodes to be forwarded (lines 2–7). For this function, only if a neighboring node advances towards the surface sonobuoys can a candidate node be considered. (Lines 3–6). A difference between the current I node sender depth and j in Pj = depth(i) - depth(j) is used to measure a packet advancement for a neighbor j. The fitness of the neighbor j is then determined (line 5). Using connection reliability, packet progress and remaining energy to assess the suitability of a nearby node. This is measured in line 5 and thus as Eq. (1).
where Pj > 0 represents packet advancement of node j; p(dj, m) is an estimation of data packet of m between node i and node j; Ejrem shows remaining potency of j; and Ejinit is the initial value of potency of j.
The nodes allowed the connection to be selected and sorted by fitness value (line 8). Finally, from the potential nodes the relation set is calculated. A limited link set can lead to low reliability of the link. At the other side, a wide connecting set may also damage the query, because it takes a long time. The possible node connections are applied to the whole collection until the required link reliability γ is reached.
B. Design of Shrewd Underwater Routing Synergy
Using Porous Energy Shells (SURS‐PES) : To transmit the data packet from source to sink-node, authors used a newly-developed link with residual energy and depth detail. In an area where energy usage has direct impact as the entire active underwater nodes rely on batteries and when cost-effective data packets are delivered, no charge or replacement steps are taken becomes a crucial factor. The authors utilized a shrewd link selection mechanism, if a link is less than or equivalent to 50% shaky, after broadcasting of a sensor node the destination node is checked, and the destination node is returned to the source node, adding some unusable porous energy shell to strengthen a link from 5% to full 90, and then transmitting it to the target.
Link Selection methodology: The link quality inspection has been taken through link reparation mechanism that is depicted in Figure 2. sensor node, a, broadcasts the packet, p, with substantial information such as depth, ID, and residual energy towards neighboring nodes, i.e., b, c, and d. The source node, Na is broadcasting the packet towards neighbors, upon receiving this packet node b includes the necessary information and sends it back as Nbp’ to node a.
When a duplicate node a is attached to the required energy shells, the packet multizes again to node b as Na2p, in a trivial time t, The grain of the final relation is measured as shown in Eq. (2).
eventually, the optimal link is being obtained holding energy utilization Eap, Ebp' and Ea2p respectively thereby remains unchanged thereupon Eq. (3), The probability of connection status from 50 to 90 percent updates in due course.
There is exhaustive study of the contact connections between node a and others. Therefore, there is a stipulated connection quality control, which records the hop links are hit by more than 50% and which links are more stable than 50% at all. Unlike the consistency of the connection between the source node a and b, the connections to the node a and d are more than 50 percent stable, but not up to 90 percent stable. The suggested approach (SURS-PES) therefore takes account of the hop connection between node a or b for the more secure packet transmission, i.e. up to 90 percent.
C. USPF: Underwater shrewd packet flooding mechanism through surrogate holding time
The authors  developed a shrewd data forwarding mechanism by taking three unique steps in regards to link selection and packet holding time namely called surrogate holding time. They implemented an angled approach in order to boost the distribution of data packets and to revitalize the life of the network. No single process consists of three stages, from source to sink. Forwarder Hop Angle (FHA) and Counterpart Hop Angle (CHA) are litigated for inclusion of data packets in the first phase of the same transmission field. If a value of FHA is equal to or higher than CHA, the packet produced will be in the same zone of transmission otherwise it would claim that the packet has other maverick. The next step selects the best relayscale node by again using the Additive-Rise and Additive-Fall method in three states connection consistency with prefix values. Ultimately, the third stage offers the exorbitant overhead fistula a definitive solution; the package holding time is built to avoid the risk of a packet loss.
Link Selection methodology: The link quality of forwarding node considering P and the neighbor nodes has been explored using Additive-Rise and Additive-Fall methods  that shrewdly makes the adjustment to the states of the Forwarding hop angle values as illustrated in Figure 3.
The aforementioned three steps are described as under:
Step 1: The forwarder node p changes the route by producing more αi packets to exploration more sparsely when the Connection status (Sh_L) is shaky or slanting compared to the prefix value (Prefix_v) with next nodes.
Step 2: If the connection state (St_L) is secure and hence meets the prefix property (Prefix_v), packet forwarding takes place without any obstacles.
Step 3: At a time if the connection state (Nr_L) is regular, but not ready for transmission due to certain salinity consequences, certain energy packets with additional shell have to go ahead and, for this reason, only fewer nodes are involved in transfers.
With relation quality only from forwarder to neighboring nodes, the flood zone is modified. Eq. (4) points, increasing node temporally updates the threshold value. A stronger connectivity also offers a slight delay. Throughout the reparation of the hop angle of the counterpart, a flood zone is never impacted by a nullity, since the hop angle is complex in hop by hop form. However, any relay node is aware of the hop angle of counterparts in the nodes around, which seem to preclude nodes from engaging in the forwarding process.