Wind tunnel experiments by the present authors have shown that a large crossflow oscillation can be induced on the upstream cylinder in cruciform cylinder system by two types of longitudinal vortices, i.e. trailing and necklace vortices. In this study, experiments on this phenomenon are carried out in a water tunnel over a Reynolds number range comparable with that of the wind tunnel experiments to investigate the effects of the mass ratio M_R and the Scruton number Sc. The alternating lift coefficient C_LR for Kármán vortex excitation in water flow agrees well with that in air flow in spite of the large differences in M_R and Sc between them. However, the value of C_LR for the trailing vortex excitation is much lower in water flow and no definite excitation due to the necklace vortex is observed.