## 1 Introduction  
Seventy percent of the world is covered by water. Therefore, for human beings, underwater environments are quite important. Moving further, underwater resources are paid attention by scientists due to the depletion of terrestrial resources. UWSNs have many uses, such as being commonly used in coastal surveillance and assurance, an expectation of sea calamity, observation of underwater contamination, military protection,road assistance, management of the aquatic oceanic climate, investigation of underwater properties, monitoring of water quality, and so on in [1; 2; 3]. Protocols designed for Terrestrial Wireless Sensor Networks (TWSN) are unable to perform efficient data transmission in the aquatic world [4]. As a result, there are differences between TWSNs and UWSNs from different perspectives, such as the use of acoustic connections rather than radio links. UWSNS topology is more difficult than TWSNs because underwater nodes travel with water streams unreservedly and adapt their positions constantly, it is more difficult to locate nodes in UWSNs than TWSNs, and node placement is often sparse [5]. Moreover, UWSNs face other challenges such as low bandwidth resulting in low underwater data rate, low propagation speed resulting in high delay. Moreover, underwater acoustic waves propagate at a speed of 1500 m/s, which is 5 orders less than radio waves, and propagation occurs over multiple paths that leads to signal distortion through a fading propagation environment [6]. Nodes underwater also face high mobility resulting in network topology instability [7]. Sensor nodes used with acoustic transmitters and receivers are placed underwater. In water, these sensor nodes may be either mobile or static. Furthermore, data is sent to surface sink nodes by the sensor nodes, often using lower depth sensors as relay stations. Autonomous Underwater Vehicles (AUVs), on the other hand, will collect data from sensors immediately. AUV routing necessitates a great deal of planning. In [8], a tracking method based on the cloud-like model data association algorithm is introduced to track underwater various targets. In comparison, the use of AUVs can incur additional costs. Throughout the UWSN system, routing protocols play a vital role. These protocols are responsible for producing suitable routes and data on those routes through sensor nodes to stations where significant data can be gathered. These protocols must be robust to extreme aquatic conditions to achieve maximum performance. These protocols, for instance, can successfully solve energy limitations, mobility of nodes, high SINR, high-end delays, etc. Following a detailed analysis of the latest UWSN routing schemes and some critical findings on the problem of the energy void, the decreased propagation ratio of the packet, and the overall load on sensor nodes closer to the sink are made. The closing remarks are as follows:  
* Most of the existing routing schemes use maximum transmission power of sensor nodes while transmitting packets to neighboring nodes or sinks. This process ends with the rapid absorption of energy by sensor nodes. In comparison, the high transmitting capacity use often produces more channel interference [9]. Consequently, the probability of energy hole creation is also increased. * Unbalanced transmission data load distribution resulted in a rapid decline in energy nodes and created network energy gaps. Owing to the energy hole occurrence, various sensor nodes die before the rest. This energy hole problem results in consuming high energy and decreases the lifetime of the network. Extra loads on nodes closer to the sink node can be removed by the existing methods. The nearest nodes to the sink consume their resources quickly due to the heavy load, while nodes farther away consume their resources slowly. Consuming energy rapidly also decreases the Packet Delivery Ratio (PDR) [10]. * Most of the existing UWSN considers the usage of one or multiple static sinks at the water surface for data collection.