To improve the highest speed and sudden acceleration performance, and obtain sudden deceleration performance even the draft position of a planing boat changes, by providing water storage tanks in the fore and rear parts in a seat respectively to control water quantity to be stored.

Rear and fore part water tanks 1(4) and 2(5) having watertight structure are arranged being enclosed by a cushion material (16) on the upper surface parts of the rear and fore end parts of a seat bottom plate (15) respectively. These mating water tanks 1(4) and 2(5) are connected by a communication pipe (13), and water, stored in the respective tank (4) and (5) insides, can be made intercommunicable by opening/closing a valve 3(8). A draft sensor memorizes the draft position of the initial rear part of a boat to adjust the flow rate of the water tank 1(4) for retaining the draft position from a draft position judging condition optimum for the highest speed and sudden acceleration.