This paper studies image contrast and limiting visibility range of a small target observed against the background of the bottom of a water reservoir under active or passive illumination. We use the small-angle approximation of the radiative transfer theory to simulate analytically multiple scattered radiation from the target tracked and that from a water medium with taking the shadowing of the medium portion by the target into account. The contrast magnitude is shown to behave 'unusually' under certain conditions and grow up with submerging the target into water having even a maximum at some depth. The physical explanation of this maximum is given and the conditions for such an 'unusual' behavior of the contrast are evaluated. On the other hand, the target with certain albedo could be invisible over all depths starting from the water surface and don tot he bottom. The case of equal albedos of the target and bottom is also considered. The target is observable here at its location depth being smaller than some threshold value. We provide an analytical estimate of this value to derive it via optical characteristics of water. In addition, the results of some case studies are presented here for different optical characteristics of water medium. The obtained data would be useful for experts in the development of optical vision systems for small targets and in the simulation of the same by advanced algorithms for sensor signal and data processing.