This paper proposes a scheme for developing a low-cost underwater acoustic communication link and implements a portable underwater wireless sensor node capable of monitoring sub-aquatic parameters and transmitting them back to a land-based base station. The designed underwater communication link utilizes ultrasonic vibrations from off-the-shelf ultrasonic transducer pairs modified to achieve underwater wireless communication. The underwater sensor node is equipped with temperature and pressure sensors, measurements from which are periodically transmitted to the surface base station. The surface base station can additionally connect to various smartphones and computers using a Bluetooth interface. The proof of concept implementation shows consistent performance under laboratory conditions.