Fast scanning is highly desired for both ultrasound and photoacoustic microscopic imaging, whereas the liquid environment required for acoustic propagation limits the usage of traditional microelectromechanical systems (MEMS) scanning mirrors. Here, a new water-immersible scanning mirror microsystem has been designed, fabricated and tested. To achieve reliable underwater scanning, flexible polymer torsion hinges fabricated by laser micromachining were used to support the reflective silicon mirror plate. Two efficient electromagnetic microactuators consisting of compact RF choke inductors and high-strength neodymium magnet disc were constructed to drive the silicon mirror plate around a fast axis and a slow axis. The performance of this water-immersible scanning mirror microsystem in both air and water were tested using the laser tracing method. For the fast axis, the resonance frequency reached 224 Hz in air and 164 Hz in water, respectively. The scanning angles in both air and water under ±16 V DC driving were ±12°. The scanning angles in air and water under ±10 V AC driving (at the resonance frequencies) were ±13.6° and ±10°. For the slow axis, the resonance frequency reached 55 Hz in air and 38 Hz in water, respectively. The scanning angles in both air and water under ±10 V DC driving were ±6.5°. The scanning angles in air and water under ±10 V AC driving (at the resonance frequencies) were ±8.5° and ±6°. The feasibility of using such a water-immersible scanning mirror microsystem for scanning ultrasound microscopic imaging has been demonstrated with a 25-MHz ultrasound pulse/echo system and a target consisting of three optical fibers.