## 4 Next Forwarding Node Selection in UWSNs
### Comparative Analysis and Discussion  
Due to the unique characteristics and harshness of an underwater environment, Opportunistic Routing is one of the most interesting routing techniques in UWSNs. Based on forwarding nodes selection, routing protocols can be divided into two categories, localization and non-localization routing protocols. However, the routing protocols in the localization category are not efficient. This is because finding location information in this category is costly due to the lack of GPS in an underwater environment. Therefore, the non-localization category is the most interesting for designing routing protocols for UWSNs. Based on data collection, non-localization routing protocols can be divided into two major subcategories, beacon-based and pressure-based. Beacon-based subcategory needs partial information provided by sink node. This information should be updated within a short space of time, which leads to high energy consumption and high overhead. Thus, beacon-based is not efficient in an underwater environment. On the other hand, in a pressure-based subcategory, the nodes are equipped with pressure sensors that are inexpensive and can calculate the depth information locally without using full or partial location information. This information can be employed for identifying the node's position and the next forwarding nodes without imposing any extra network overhead. More precisely, finding the next forwarding nodes in the pressure-based subcategory does not require full location information or partial sink information. Therefore, the opportunistic pressure-based routing subcategory is the most promising and interesting routing method for UWSNs. Hence, this study focuses on designing an energy-efficient and reliable opportunistic pressure-based routing protocol for UWSNs. In this paper, we have reviewed the most common routing protocols in UWSNs regarding selecting the forwarding nodes. We have summarized the existing well-known routing protocols in Table 1. In this table, the comparison between various routing protocols in UWSNs is simplified. The significance of each column is explained in the following. * **Category and Protocol**: this field identifies our introduced categories and the names assigned by authors to the proposed protocols belonging to each category. The corresponding references are also provided here. * **Objectives**: this field shows the main goal of the proposed algorithms, such as energy efficiency, reliability, and void handling. * **Modeling parameters**: we have extracted the main metrics utilized in the existing routing protocols; each protocol uses different metrics in terms of selecting the next forwarding nodes. * **Neighbor selection strategy**: the aim of this paper is to analyze the main issues in UWSNs regarding selecting the next forwarding nodes. Therefore, this column highlights the main techniques of how each protocol selects the next forwarding nodes. * **Forward selection strategy**: this column points out how each protocol selects the best nodes among the candidates. Table 2 provides full performance factors of the protocols, which includes a summary of the main behaviors of all protocols discussed above. This summarization is based on dependent factors used in selecting the next forwarding nodes, including residual energy, link quality, void-aware, reliability, multi-hop, shortest path, and sender/receiver-based.