This paper reports the development of a compact optical-acoustic frontend module for underwater communication and ranging. The module is enabled by a new water-immersible MEMS scanning mirror (WIMSM). It is capable of transmitting, receiving and steering co-centered and co-directional laser and ultrasound beams under water. To monitor its rotating angle in real time, scan position sensors based on Hall effect have been integrated into the WIMSM. The angular alignment of the laser and ultrasound beams in both transmission and reception modes has been examined. The experimental results show that the laser and ultrasound beams can remain aligned with less than 2.1 degrees under envelope of pan and tilt rotations. This capability is critical for the continuing development of the new bi-modal communication and ranging underwater Vehicles (AUVs).