If you really have an infinite amount of water, then the energy in the water is infinite: your water pumping device always uses energy, and that's not a " perpetual motion machine" which only produces energy but does not. input energy. Moreover, in real situations, there cannot be infinite water, so it is even more impossible.
If there is a continuous supply of water to replenish it, it is equivalent to replenishing water energy - it cannot be regarded as a "perpetual motion machine". As for its exploitation - hydroelectric power stations, tidal power stations all exploit this phenomenon to produce electricity - it has already been exploited.