Recently, ROV and AUV are often used in the maintenance of the offshore structures and the development of the ocean resources. These operations are carried out with underwater manipulator of the robot. As the energy consumption of untethered type ocean robot is limited, it is required to save energy during operation. But in order to calculate the target trajectory for minimum energy consumption, precise mathematical model of the dynamics of the manipulator is required. In this paper, we try to apply the Learning Feed-Forward Controller (LFFC) to identify the dynamics of the controlled object, especially friction force acting on the joint. Using the identification results, we formulate and calculate the trajectory of the minimum energy consumption, then examine the effect of the trajectory planning method by model experiments. As a result of the study, it is shown that the dynamics of the controlled object can be identified precisely with LFFC, and that the trajectory obtained from the solution of the two point boundary value problem is nearly equal to the actual trajectory of minimum energy consumption.