Seaplane design requires a delicate balance between aerodynamic efficiency for flight and hydrodynamic performance for water takeoff. This study presents a CFD-based optimization of a seaplane configuration, focusing on the coupled aerodynamic and hydrodynamic characteristics during takeoff. The main geometric parameters are listed in Table 1. The MH114 airfoil is optimized using Isight and Xfoil, achieving a 15% increase in maximum lift-to-drag ratio. As shown in Fig. 1(a), CFD simulations reveal that as the angle of attack increases from -2° to 1°, the lift-to-drag ratio rises steadily, reaching a maximum of 19.1 at 1° angle of attack with a cruise speed of 23 m/s. Beyond this angle, the lift-to-drag ratio gradually decreases, dropping to 18.4 at 3° and 17.1 at 6°. The static margin is 20%, ensuring longitudinal stability, with a stall angle of 16° and a maximum lift coefficient of 2.0. Self-propulsion simulations using overset mesh and 6-DOF motion reveal that as shown in Fig. 1(b), as speed increases from 3 m/s to 9 m/s, the planing resistance rises progressively, reaching a peak of approximately 150 N at 9 m/s, accounting for 15% of the takeoff weight. Beyond this speed, the resistance decreases rapidly as the hull lifts off the water, dropping to 107 N at 11 m/s and 68 N at 13 m/s. The step creates a pressure drop region that effectively reduces the wetted area. The lift-off pitch angle is 8.2°, which matches the afterbody angle of 7.5°, ensuring a smooth transition from water to air. Three longitudinal center-of-gravity positions (forward, mid, aft) are evaluated to investigate their effects on planing stability. Results show that the mid-CG configuration achieves the best balance between stable planing attitude and peak resistance.