## 1 Introduction
In addition, some of the surveys do not address the parameters such as routing strategies of the addressed protocols or their merits and demerits. A description of these parameters is essential for the researchers, engineers and scientists that design and test the routing algorithms for UWSNs. The description of these parameters helps in the selection of the right protocol for the right application in UWSNs. Moreover, these parameters also help researchers to design new routing strategies based on the addressed flaws in the addressed routing schemes. This, in turn, leads to the foundation of new routing protocols that are more robust, intelligent, efficient and smart as compared to the addressed protocols. In this paper, a survey of the routing protocols for UWSNs is given. As compared to the existing surveys, this survey considers only recent and state-of-the-art protocols (designed in the last one or two years except some pioneering protocols). The protocols are classified into two categories: localization-based and localization-free protocols. The former type requires the two- or three-dimensional position coordinates information of the sensor nodes, while the latter type requires only the pressure of the water on the sensor nodes (depth) to identify the routing trajectories. Further division of the two categories of protocols is into multiple subcategories based on the problems they address or the major parameters they consider during routing. The subdivision includes categories based on the nodes' mobility, energy balancing, channel properties, energy consumption and the void zone. This division and subdivision are helpful and effective for researchers to understand the theme of the design of every protocol. Unlike the existing surveys, the survey particularly focuses on the routing strategies, merits and demerits of the addressed protocols. The description of the routing strategies helps researchers understand the routing process of the addressed protocols. The description of the merit(s) of each protocol eases its selection by researchers for a particular application, depending on the nature of the application. For instance, in time-sensitive applications, such as military surveillance and disaster prevention, a protocol selecting the shortest path to shorten the propagation delay is the most effective as compared to other protocols. In a similar fashion, applications such as underwater pollution detection and equipment monitoring (where time efficiency is not desired) require that the network operates for a significantly long time. Therefore, the selection of energy-efficient routing protocols suffices for the requirements of these applications. Describing the demerits of the protocols is useful in the design of new routing protocols for UWSNs that are more effective. This is because new protocols can be designed keeping in view the demerits of the existing protocols. In this way, more robust, efficient and smarter protocols can be designed. Moreover, the challenges associated with the underwater medium are addressed, and the protocols are described in terms of addressing these challenges, as well as the challenges that arise during packet forwarding. Finally, open research challenges and future research directions are highlighted for the researchers, scientists and engineers. In essence, the following are the contributions of this paper:  
* The current and state-of-the-art routing protocols for UWSNs that are mostly not addressed in the existing surveys are considered. Unlike the existing surveys, every protocol is described in terms of its routing strategy, merit(s), demerit(s) and latency. The merit(s) of every protocol is (are) linked with the cost. For instance, the high delivery of packets to the final destination of some protocols is linked with their high energy consumption. This, in turn, leads to a short network lifetime. The routing strategy of every protocol is described to help researchers understand the protocol conveniently.