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Optical and sonar cameras are suitable imaging systems for inspecting underwater structures, both in regular maintenance and security operations. Despite their high resolution, optical systems have limited visibility range when deployed in turbid waters. In contrast, the new generation of high-frequency (MHz) forward-scan sonar cameras can provide images with enhanced target details in highly turbid waters though their range is reduced by one or two orders of magnitude as compared to traditional low or midfrequency (10s-100s KHz) sonar systems. It is conceivable that an effective inspection strategy is the deployment of both optical and sonar cameras on a submersible platform to enable target imaging in a range of turbidity conditions. Under this scenario and where visibility allows, registration of the images from both cameras (arranged in binocular stereo configuration) provides valuable scene information that cannot be readily recovered from each sensor alone. We explore and derive the constraint equations for the epipolar geometry and stereo triangulation in utilizing these two sensing modalities with different projection models. Theoretical results supported by computer simulations show that an opti-acoustic stereo imaging system outperforms a traditional binocular vision with optical cameras, particularly for increasing target distance and/or turbidity.