For water waves, the nonlinear Schrödinger equation describes the evolution of the envelope of modulated wave groups. In a paper in 1968, Vladimir E. Zakharov describes the Hamiltonian structure of water waves. In the same paper Zakharov shows, that for slowly modulated wave groups, the wave amplitude satisfies the nonlinear Schrödinger equation, approximately. The value of the nonlinearity parameter "к" depends on the relative water depth. For deep water, with the water depth large compared to the wave length of the water waves, "к" is negative and envelope solitons may occur. Furthermore, these envelope solitons may be accelerated under external time dependent water flow.