## 1 Introduction  
The planet Earth consists of 70% water. This shows the importance of exploring the underwater medium. The sensor nodes in UWSNs cover a specific area of the sea to sense some attributes and inform the onshore data center near the water surface [1]. In forwarding data packets from the bottom to the surface of the water, the sensor nodes communicate with one another to identify and use the paths that are the best with respect to some selection criterion(a). Routing protocols for UWSNs deal with the selection of such paths to deliver data packets to the surface destination in an efficient and effective fashion. Recently, researchers, scientists and engineers have used the routing protocols to investigate the underwater medium for a number of applications. They include monitoring of the underwater environment for military and civilian purposes, forecasting disasters, leak detection [2] and general underwater exploration [3]. The underwater medium is highly challenging and unpredictable due to a number of reasons [4]. Water has the property of absorbing radio frequency (RF) waves. This increases the attenuation and causes energy loss of RF waves in water. For a specific frequency \(f\) in Hz (Hertz), the rate at which water absorbs the energy of the radio waves is \(45\sqrt{f}\,\mathrm{dB/km}\) (decibels per kilometer). Water becomes a conductor of the RF waves at extra low frequencies in the range of 30-300 Hz. However, this frequency range requires a very large antenna size for the transmitter with a high transmit power. This requirement is impractical and, consequently, the RF waves are not used in underwater communications. The use of the optical waves is also not effective in the underwater environment. This is because the optical waves require high precision to point them between a transmitter and a receiver, while the underwater medium is unpredictable and sensor nodes move with the water currents. In essence, acoustic waves are used in UWSNs. However, the speed of acoustic waves is almost five-times in magnitude slower than the speed of RF waves. This causes high propagation delay in underwater communications. The use of acoustic waves limits the range of useful frequencies in underwater communications as the acoustic spectrum is limited [5]. Furthermore, the speed of an acoustic wave changes significantly with depth, salinity and temperature of the water. This causes the acoustic waves to travel on curved paths in water, which as a result, creates zones where sensor nodes are inaudible. Such sensor nodes then do not participate in the data transmission process, which affects the performance of the network. The marine life, shadow zones and unpredictable nature of the underwater medium with high interference and severe noise further threaten the reliable delivery of data packets from the bottom to the surface of the water. Moreover, the sensor nodes have limited battery life, and it is inefficient to replace the batteries of the sensor nodes [6], especially at the ocean's bed. The design of routing protocols for UWSNs is of paramount importance. These protocols identify paths from the bottom to the surface of the water to ensure network performance in accordance with the desired parameters. Specifically, the challenges associated with the underwater medium and during packet forwarding are considered by these protocols to achieve the optimal performance of the network according to the desired goals. For instance, these protocols cope with the limited battery power, severe noise and interference, shadow zones, movements of the sensor nodes with water currents, reliable delivery of data packets during unfavorable channel conditions and high propagation delay [7]. There are many surveys conducted on routing protocols for UWSNs in the literature [8; 9; 10; 11]. However, these surveys are either not too focused on routing protocols or do not consider the recent routing protocols for UWSNs.