## 6 Conclusions, Open Issues and Research Directions  
In this work, we have conducted a review investigating different factors that affect cooperation in UASNs. We study various techniques applied in UASNs from different perspectives and we present a taxonomy for underwater cooperation. We further analyzed different game methods and compared their performance based on different metrics in which our analysis shows that a cooperative game theoretic based approach is the most suitable and promising method for providing cooperation in UASNs. Games will let the network nodes decide the best options based on interaction through game rules, so a malicious sensor would need to interact using the game rules, and that interaction would not necessarily result in a successful transaction. In contrast, reputation- and price- based techniques would not be so effective in scenarios where malicious nodes come and steal identities from good nodes, since they only need to announce their reputation or price and interchange messages to be considered as reputable nodes. The competition included inherently in games would make it difficult for a malicious node to get trust from other sensors. Competition brings the best result for all the players, that is why the use of game theory provides a platform for competition under a framework of rules, whereas the other two techniques (reputation-based and price-based are based on bonuses or punishments, thus creating an environment where individuals try to excel (having the highest reputation, or having the best price of a service to be used by others), not necessarily in a cooperative way that benefits more individuals. Game theory on the other hand creates these environments where cooperation achieves a common goal or network objective without the interest to excel individually. Through our literature review, we have found that none of the current research considers all the performance metrics that are necessary for an optimum network performance. There are many aspects or components which support and contribute to better achieving cooperation performance in UASNs, see last column of Table 2. Therefore, the exploration of those components is yet to be addressed in future research as an integral part of the cooperative game. A suggestion of some of the necessary aspects of the network that need to be included in cooperative game methods and need to be studied further are:  
* _Reliability:_ This is one important aspect to have a reliable successful forwarding and delivery of information among participating players of sensor node in UASNs. It is a key point that guarantees various forms of reliability such as data reliability, hop-by-hop reliability and end-to-end reliability. Cooperative game methods need to consider strategies such as information protection, packet repetition, route redundancy and maintenance, to provide reliability. This means that cooperative methods to be developed need to be extended to include reliability issues such as those mentioned at the same time of performing cooperation. Reliability guarantees a successful delivery of data between players participating in cooperative or collaborative activities. The investigation in this research found that this very important key component of cooperation is missing in most of the current literature in UASNs, and therefore, it is required to come up with a cooperative mechanism that will take this reliability into consideration. * _Efficiency:_ Efficiency is strongly required in a communication network to provide an efficient cooperative method and to facilitate cooperation among players. Based on our investigation of the current work, it has been found that there is no scheme, method, approach, or mechanism which takes into account this aspect. Cooperative or collaborative monitoring activities require an efficient mechanism for a successful packet forwarding and delivery in UASNs.