A positional sensor detects a current position and a current bearing of a boat body and outputs position information indicating the current position and the current bearing. A controller receives the position information. The controller determines a first target position of the boat body and a target bearing in the first target position. The controller calculates a bearing difference between the current bearing and the target bearing. The controller determines an offset amount of the first target position in response to the bearing difference or an outside disturbance. The controller determines a position spaced away by the offset amount from the first target position on a side toward the current position as a second target position. The controller generates an instruction signal to control a propulsion device so as to cause the boat body to arrive at the second target position.