During cruise, at an altitude of about 700 feet AGL, the airplane's engine started to sputter and lose power. The pilot stated that he switched tanks from the left fuel tank to the right fuel tank, and about 15 minutes later the airplane continued to lose power. He stated he tried to switch fuel tanks again, but the engine did not respond and he could not maintain the airplane's altitude and was forced to ditch. He stated that the engine never lost total power. At the time of the accident, it was lightly snowing. The outside air temperature was 32 degrees F and the dew point was 29 degrees F. Examination of the airplane did not reveal any mechanical malfunctions. The fuel tank selector was found selected to the left fuel tank. The left fuel tank was empty and the right fuel tank had an undetermined amount of fuel in it. The carburetor heat control was found in the 'cold' position. At the time of the accident, according to the icing probability curves, conditions favorable to the formation of induction system icing existed. The pilot stated in the recommendations section of the pilot report, '...pull the carb heat...carb ice should be given more emphasis in training....'