The important bit is "under load". Any motor will take a given amount of power to just turn itself over, the rest of the power draw is down to the load, in the case of a boat, this is determined by the prop in the water. Different sizes and configurations of prop will give different results, and the requirements for different types of performance means that there is a vast range of possible combinations. The only reliable way is to do empirical testing. Without an ammeter (and common ones are not usually rated above 10A) the simple down and dirty way is to power the motor direct from a battery with a fuse in circuit. Use higher values until you get the lowest value that doesn't blow.