The Teensy audio adapter [1] uses a dedicated DAC, the SGTL5000. That frees up the MCU to do more DSP.
Radio signals below 50 kHz are capable of penetrating ocean depths to approximately 200 metres, the longer the wavelength, the deeper. The British, German, Indian, Russian, Swedish, United States [3] and possibly other navies communicate with submarines on these frequencies.
->That requires min 100khz sample rate.