I finally found the solution myself. I really feel stupid now. It is so obvious, if you know it:
Without IAAM: The sun is always at one's right-hand side. No matter how you rotate the camera, i.e. no matter in what cardinal direction one looks, the rays of sunlight always come from the right. Of course, this is not natural, because this means the sun rotates, too, if the observer rotates. However, one constantly has sun from the side, one sees some reflections and the water always looks "beautiful".
With IAAM: The sun is steadily located at the south. This is probably the more "natural" way. However, this means, if one looks northwards (which is the default orientation) everything looks "flat", because the sun is at your back: no shadows of building (they are hidden behind the buildings), no reflections in the water. However, if one rotates, the incidence angle of the sun changes accordingly. If one looks towards east, one has the "original" setting like in ANNO without IAAM. If one looks south, one even has reflections all over the water, because one directly looks into the sun and is dazzled by sun.
Attached you find two screenshots from the harbor of my capitol. The first one is taken looking into the "default" direction, i.e. northwards. The second one is taken looking southwards: voilà, reflections are back :D. Also note the correct direction of shadows of the building.
PS: But honestly, nobody has noticed this change before me?! oh, oh.