The aircraft was flown to Hoxton Park and a number of circuits and landings were carried out as part of an endorsement exercise. The pilot in command then elected to return to Bankstown in order to refuel the aircraft. During the approach the engine lost power and attempts to restart it were unsuccessful. A ditching was carried out in the Georges River and the pilots swam to the shore. The aircraft had been cross-hired and the instructor, who had not previously flown the aircraft, told the student to conduct a preflight check while he familiarized himself with the avionics. However, the student did not check the fuel contents because he could not reach the fuel caps and did not carry out a water check because he was unaware of the location of the fuel drain points. The instructor said that before the engine lost power he had selected the left fuel tank because the right tank gauge indicated empty. After recovery of the aircraft an examination revealed no prior fault with the engine or the fuel system. The fuel system was contaminated with river water and the right tank contained no usable fuel. However, the left tank contained 16 litres of usable fuel which would have been sufficient to provide engine power with the left tank selected while the aircraft was in level, co-ordinated flight. It is probable that the initial power loss was caused by fuel starvation resulting from uncovering of the left fuel tank outlet. The instructor's attempts to restart the aircraft were not in accordance with the manufacturer's checklist and included 5 movements of the fuel tank selector in quick succession following the loss of power.