Marine impacts can develop a crater in the seafloor if the target water depth is shallow in relation to the size of the impactor. The geology of this marine-target crater is influenced by the layer of water in the upper part of the target. The influence increases with increased water depth. The target water depth is essential when calculating the magnitude of the impact event as the seafloor crater merely expresses some of the expended energy. The target water depth for a marine-target crater is often estimated by facies analysis of sediments related to the impact crater, unfortunately often with ambiguous results. We propose to combine the conventional methods with numerical modeling of the target water depth based on the special target-water-related features of the crater. We used geological data from the Lockne crater as constraints in a simulation with the SOVA hydrocode. The simulations were done for three different target water depths (200 m, 500 m, and 1000 m) within the likely depth range for the Middle Ordovician sea, in which the impact occurred. In order to obtain the minimum estimated size of the Lockne crater by an asteroid impact at 20 km/s, the water depth must have been slightly less than 1 km, and the impactor radius must have been about 400 m. It was not possible to generate a crater with the geological features of Lockne for target water depths of less than about 500 m. However, the ratio may be further constrained by factors such as impactor density, impact angle, and impact velocity.