Abstract: The distribution of the added water mass is investigated for an immersed cantilever cylinder through an analytical method. A non-dimensional coefficient for the mass distribution is derived. With a numerical fitting analysis of the mass distribution, the effect of the added water mass can be expressed as the equivalent mass density of the structure. The mass density of the immersed cylinder is replaced with the equivalent mass density in the vibration analyses. The wet frequencies of the immersed cylinders are experimentally studied. The wet frequencies obtained by the present simplified (added water mass) method agree well with the experimental results. It is shown that the mass effect of the around water can be well described by the present simplified formula.