This paper presents an algorithm for image registration and mosaicing on video sequences acquired by an underwater acoustic camera. The sonar images of our interest can be characterized by a high noise level, inhomogeneous illumination and low frame rate. Imaging geometry of acoustic cameras is significantly different from that of pinhole cameras. For a planar surface viewed through a pinhole camera undergoing translational and rotational motion, registration can be obtained via a projective transformation. We show that, under the same condition, an affine transformation is a good approximation for an acoustic camera. We introduce image fusion algorithms which reduce the geometrical distortions which are caused by sharp camera movement. This reduces image blur and increases image resolution.