Results of calculations of the amplitude and shape of a signal of a spaceborne lidar operating in the system atmosphere–ocean are presented in the paper. The calculations have been done by the Monte Carlo method for wavelength λ=0.53 mm. The dependence of the lidar return signal characteristics on the atmospheric optical state, water–air interface, and optical properties of sea water has been studied. A signal from the atmosphere is shown to mask completely a signal from a subsurface water layer.