Investigating and safeguarding our oceans is vital for a host of applications and tasks, including combating climate change, ensuring the integrity of subsea infrastructures, and for coastal protection. Achieving these essential functions depends on the deployment of cost-effective, versatile underwater sensor networks that can efficiently collect and transmit data to land. However, the success of such networks is currently hindered by the significant limitations of existing underwater modems, which limits their operational use to a narrow range of applications. This paper presents and evaluates the performance of the SEANet software-defined networking platform, for the Internet of Underwater Things (IoUT), addressing the limitations of existing underwater communication technologies. It presents the development and comprehensive testing of an adaptable, high-data-rate, and integration-friendly underwater platform that reconfigures in real-time to meet the demands of various marine applications. With an acoustic front end, the platform significantly outperforms conventional modems, achieving more than double the data rate at 150 kbit/s. Experiments conducted in oceanic conditions demonstrate its capabilities in channel characterization, OFDM link establishment, and compatibility with the JANUS communication standard. Our platform advances the IoUT by providing a versatile, scalable solution that can incorporate multiple physical layers and support an array of tasks, making it pivotal for real-time ocean data analysis and the expansion of ocean-related digital applications.