A low-power, low-latency, and long-range (LoRa) relay is developed using mixed-frequency underwater wireless communication systems (UWCSs) for LoRa wide-area network. First, the feasibility of utilizing LoRa in an underwater-to-above-water communication link is examined through theoretical and experimental approaches. In addition, the communication link quality is evaluated using the received signal strength indicator, signal-to-noise ratio, and packet delivery ratio at different depths, horizontal distances, and LoRa physical layer parameters. Finally, considering the experimentally observed challenges of UWCSs, the LoRa-based relay is designed, and its usability is validated by comparing the communication systems without and with the relay (from underwater to the water surface to the above-water) in terms of LoRa physical layer parameters, transmission range, and link quality indices. Moreover, the network latency and battery consumption of the whole system are studied without and with the relay using different LoRa physical layer parameters, and the optimum parameters are presented. The results show that the designed relay can improve communication performance without delay and power consumption concerns.