Why does an unloaded D.C. motor draw significant current?
There is not enough information available to do more than guess at how the motor and pump are performing. The pump manual indicates the pump can pump at 20 gallons per minute with a 3.45 ft. (1 m) static head and 11.33 GPM with a 6.7 ft. (2 m) head. That is 12.6 watts and 14.3 watts actually used in lifting the water.

The motor is designed to cycle on and off frequently. Every 2.5 minutes the pump is supposed to run for a short time to detect the water level. When the motor is first turned on, the only thing limiting the current is the winding resistance. If the motor has a low winding resistance the current would be quite high and frequent starting would be hard on the commutator and brushes. It seems likely that the motor has a high winding resistance to limit the starting current. That means it is quite inefficient and doesn't draw much more current at maximum load than at minimum load.

Since there is probably some water in the pump most of the time, there will be some loss in stirring the water when there is no discharge. For all operating conditions, more than half of the input power is probably dissipated in the winding resistance.