Existing underwater acoustic networking platforms are for the most part based on inflexible hardware and software architectures that can support mostly point-to-point, lowdata-rate, delay-tolerant applications. Most commercial devices do not provide neither the sufficient data rates nor the necessary flexibility to support future underwater networking applications and systems. This article discusses a new high-data rate software-defined underwater acoustic networking platform, SEANet G2, able to support higher data rates (megabit/s data rates are foreseen over short range links), spectrum agility, and hardware/software flexibility in support of distributed networked monitoring operations. The article reports on the main architectural choices of the new platform, as well as some preliminary performance evaluation results. Data rates in the order of megabit/s were demonstrated in a controlled lab environment, and, for the first time to the best of our knowledge, data rates of 522 kbit/s where obtained in sea trials over short horizontal links (e.g., 10 m) for a BER lower than 10 −3 .