The work presented here describes an experiment designed to determine the performance and capabilities of ground-penetrating radar (GPR) over water. The main factors that are addressed are: water conductivity and skin depth, accuracy of depth determination, effect of raising the antenna above the water surface plus the radiation pattern and its influence on imaging an inclined submerged interface. This study was undertaken for different frequency GPR antennas which were pulled across the wavetank at Edinburgh University. Two different targets, a metal bar and a plastic plate, were submerged in the tank and the reflections of the electromagnetic signals were recorded. The objective of this work is to clarify the interpretation of GPR images so that field data may be more accurately assessed. Practical applications which could benefit from this work include the investigation of scour around bridge piers and abutments and bathymetric surveys of ponds and lakes.