Finding tie or GCP points over water is problematic for any images. If you have a base image, that is a good start. Collect several points where there are land features where you can find coincident points. If you have fiducials, do you also have the corresponding locations for those? If so, those can be used for the corners. Since water is pretty flat, those areas don't need a lot of ground control/tie points so if you can set up the corner points using the fiducial information and a few tie points on land features, you should be able to achieve a fairly decent registration result. Mosaicking these results could still be challenging. You can first try the Seamless mosaic to see how well they align. Otherwise, you may need to use pixel-based mosaicking which allows you some ability to position the images.