Electronic relaxation dynamics of Ni2+ -ion aqueous solution is investigated using molecular-dynamics (MD) simulations with the model-effective Hamiltonian developed previously. The nonadiabatic transition rates from the first three excited states to the ground state are evaluated by the golden rule formula with the adiabatic MD simulations. The MD simulations with the fewest-switch surface-hopping method are also carried out to obtain a more detailed description of the electronic relaxation dynamics among the excited states. We found out that the transitions among the three excited states are very fast, in the order of 10 fs, while the transition between the excited and ground states is slow, about 800 ps. These findings are consistent with the time scales of energy dissipation detected by the transient lens experiment. In both simulations, we explore the effects of the quantum decoherence, where the decoherence functions are derived by the energy-gap dynamics with the displaced harmonic-oscillator model.