In a wave energy converter (WEC) for capturing energy contained in surface waves on large bodies of water, one or both of the floats used in the WEC has an internal spring system including a weight suspended from a spring mechanism having a selectively variable spring constant for allowing vertical oscillations of the weight in response to bobbing motions of the float The mechanical impedance of the system is a function of the oscillations of the weight, hence the system is tunable to selected values of mechanical impedance for optimal coupling with the prevailing waves.