We introduce an algorithm for image registration and mosaicing on underwater sonar image sequences characterized by a high noise level, inhomogeneous illumination and low frame rate. For a planar surface viewed through a pinhole camera undergoing translational and rotational motion, registration can be obtained via a projective transformation. For an acoustic camera, we show that, under the same conditions, an affine transformation is a good approximation. We propose a novel image fusion, which maximizes the signal-to-noise ratio of the mosaic image. The full procedure includes illumination correction, feature based transformation estimation, and image fusion for mosaicing.