Underwater acoustic communication (UAC) networks are becoming a hot spot for research due to the growing demand for ocean development. Previous acoustic modems were mainly designed for the field of ocean engineering construction and scientific investigation, and consequently, with the characteristics of large size and high-power consumption, a low-cost, low-power consuming micromodem is proposed in this paper to satisfy the requirements of underwater Internet of Things (IoT). The STM32F767 processor is embedded in the micromodem as the core for fast digital signal processing. Furthermore, a convolutional code–block interleaving–frequency hopping–MFSK underwater communication scheme is also discussed to ensure the communication quality. Experimental results are shown to demonstrate our modem can realize the reliable UAC transmission of 200~300 bit/s at the distance of 500 m.