If the motor is in the boat and connected up to the propshaft with a prop, my method would be to pop it into a test tank.. your bath... Hook up the motor to the battery via an ammeter, this needs to be in series with the motor so the power goes from the battery through the meter to one side of the motor and the other wire goes back to the battery. The meter will give the reading of the max current drawn.
Hang on to the boat, maybe a two man job depending on the boat and motor.
Another way would be to replace the meter with a selection of fuses to find which blow or not. IE start with say a 5 amp, next a 10 amp etc