I'm not sure if I understand the difference between a counterpoise and a ground plane. Is the following correct?
A counterpoise is used as a substitute when direct earthing is not available, to increase the potential difference through capacitance. That I presume can only be used on land based antennas?
A ground plane is used for reflecting the radio waves. For example, if a 1/4 wavelength antenna is used, a 1/4 length ground plane is used to give a 1/2 wavelength.