Recently monitoring of sea healthiness using voluntary observation ships has been carried out. In the monitoring, seawater is sampled through an inlet located on a ship hull, and the sea healthiness is evaluated through analyzing sampled water chemically and biologically. In the coarse of analyses, the information of original position of sampled water is one of important factors to assess the sea healthiness, however its quantitative evaluation has not been carried out. The objective of the research is to develop the method to estimate the original depth of sampled water. In order to do that, first of all, we use a model ship experiment. In the experiment, we inject dye from the upstream of the ship and pick the water up from the inlet, and then dye concentration in the sampled water is analyzed using a spectrophotometer. The concentration is mapped onto the position of dye injection and we thus show the original location of sampled water as a contour map on the upstream sectional plane. Further, numerical simulation of flow around ship is done and by solving a diffusion equation, upstream location of sampled water can be evaluated numerically and we show that numerical evaluation agrees with the results of experiments.