This paper presents the design and performance evaluation of the SEANetplatform, a software-defined acoustic modem designed for enhancing underwaternetworking and Internet of Underwater Things (IoUT) applications. Addressingthe limitations of traditional acoustic modems, which suffer from low datarates and rigid architectures, SEANet introduces a versatile, adaptiveframework capable of reconfiguring all layers of the protocol stack inreal-time to accommodate diverse marine applications. The platform integrateshigh-performance, wideband data converters with modular hardware and softwarecomponents, enabling real-time adaptation to changing environmental andoperational conditions. Experimental evaluations conducted in oceanic settingsdemonstrate the SEANet capability to significantly exceed the performance ofexisting commercial underwater modems, supporting data rates up to 150 kbit/sand effectively doubling the performance metrics of conventional systems. Ourrobust testing also highlights the SEANet proficiency in channel estimation,Orthogonal Frequency-Division Multiplexing (OFDM) link establishment, andinteroperability through the JANUS communication standard. Our resultsunderscore the SEANet potential to transform underwater communicationtechnologies, providing a scalable and efficient solution that supports highdata rate applications and fosters the expansion of IoUT deployments.