Underwater (UW) communication technologies have progressed not only with the development of advanced coherent modulation, demodulation, coding and decoding techniques but also in moving from point-to-point systems to multi-hop ad-hoc networks. At higher communication stack layers there have been significant advances in developing DTN, MAC, routing and other protocols to establish efficient and reliable communications. It is also becoming clear that the UW bandwidth is so limited that there will never be a 'one-size-fits-all' solution, so that communication systems will need to adaptively reconfigure themselves to the (changing) network topology, environment and application. This is leading towards intelligent software-defined modems with a high degree of cross-layer connectivity. In recent years non-acoustic communication modalities have been explored and implemented into commercial products while significant progress has been made in establishing the first acoustic underwater digital communications standard - JANUS. Additionally, security in underwater communication environments is gathering more attention as the number of autonomous assets being deployed increases. Building the next generation multi-modality, adaptive, secure and interoperable underwater communications system is undoubtedly a wish of all in this field. Yet some of the hard problems remain: The physics of the channel is very rich and difficult to fully capture in simulation models. Some of these models specialize in capturing a certain degree of complexity that may suit some applications but still no common library of models has been widely adopted to benchmark new solutions. This conference aims to bring together key people in UW communications networking to review the state-of-the-art and share understanding of performance constraints and trade-offs with a view to catalysing consensus on standards for interoperability.