There is an increasing demand for underwater communication, not least manifested in a need to distribute and retrieve data from networks of underwater sensors.  Whilst there are exceptions, acoustic techniques are generally the only viable means of communication. However, transmitting information acoustically is energy-intensive and can limit the lifetime of battery-powered platforms. Through simulations, this paper statistically investigates a recent transmission power controller, developed for underwater networks of static, battery-powered modems.  The controller is self-configuring, as the modems' locations are assumed to be unknown. Further, the controller is fully distributed for scalability and adaptability reasons. The method involves a $k$-nearest neighbor approach when selecting transmission power for packet forwarding, i.e., the transmission power is selected such that only the $k$ closest modems will receive a packet. A well-known flooding-based routing protocol suitable for ad-hoc networks is employed to assess the energy consumption with and without the power controller. The evaluation is based on simulations using 16 modems placed randomly in a square area with varying sizes and choices of $k$. The results show that in a small and dense network, up to 61-68\% energy can be saved with a minor 7\% drop in packet delivery ratio. 

In this study, we present the first iteration of DPower, an energy conserving method for use in underwater acoustic networks. The method encompasses a straightforward transmission power calibration procedure and adaptive power level selection. The method was evaluated in combination with DFlood, a known and validated constrained flooding protocol developed for underwater applications. Simulations of a network with given prerequisites have shown that, with an acceptable increase in packet loss, the presented method can dramatically reduce the energy consumption and thus improve the life-time of networks. 

The underwater acoustic environment is known for its unpredictability, making it challenging to establish configuration parameters for acoustic modems before network deployment. When the modems are configured for robustness, potential throughput is often sacrificed; meanwhile, opting for high-rate links can result in communication failures in highly dynamic acoustic conditions. Given these challenges, this paper presents an adaptation framework for networked underwater acoustic modems. Its primary objective is to let modems adaptively select communication links that balance information rate and reliability. It is assumed that the modems provide a set of pre-configured links with monotonically increasing information rate and decreasing reliability. The framework is developed specifically for flooding-based routing protocols, which efficiently handle sudden changes in network topology. By leveraging existing network traffic and implicit acknowledgments, the framework achieves link adaptation with minimal network overhead, necessitating only the addition of a "previous node" address field in the packet headers. Field experiments conducted in a time-varying acoustic environment, using modems configured with four different links, show an increase in the average information per packet by a factor of up to 12, and a reduction in network transmission time of 25\%--50\%, demonstrating DLink's ability to enhance channel utilization significantly, outperforming configurations that rely solely on robust links. This improvement indicates DLink's potential to substantially increase the throughput of underwater acoustic networks.

Systems of heterogeneous underwater vehicles and sensor platforms are being used in various applications. Although these systems are still deployed at a relatively small scale, rapid technological development is in progress, and a significant impact of these systems is approaching. To a large degree, this is driven by a desire to utilize the oceans for sustainable food, energy, and environmental monitoring. Vehicles and platforms in these systems communicate wirelessly using acoustic signals due to the limited range of radio signals. To form underwater networks, the systems must communicate at the same frequency range and use common network protocols. Flooding-type network protocols are easy to implement, robust, and can serve as a baseline for heterogeneous networks. Here, a transmission power controller used in conjunction with a flooding protocol is examined. Results show a great reduction in transmission power and potential for significant energy savings.

This paper proposes a distributed, JANUS-based protocol that enables an underwater acoustic network to reach consensus on arbitrary local opinions as numeric state variables. 

An envisioned scenario where nodes shall agree on parameters describing the acoustic environment is used to evaluate the protocol. The scenario exemplifies the protocol's potential in future applications where nodes use the environment description to decide on appropriate modulation and coding schemes. The evaluation is based on simulations and sea experiments in a challenging acoustic environment. The simulations allowed examining the performance for different parameter values regarding the timing of transmission events and state transitions in the finite state machine implementation of the protocol.

The best parameter configuration was used in the following experiments conducted in a bay in the Baltic Sea. The experiments comprised several deployments of five to six commercial modems. 

Results from the experiments show that the protocol can achieve a consensus up to 89\% of the time in the tested environment, and up to 96\% of the time if the state variables are permitted to differ by one discretisation step maximum across the network. In addition, when the network separates due to environmental conditions, connected components appear to achieve consensus more often when the links are more reliable, with individual nodes with poor connectivity having a major negative impact on the probability of achieving consensus. Finally, it is shown that when different consensus processes are active in parallel, packets from one process do not interfere with the opinions in different processes.