Recently, cross-medium flight vehicles have attracted much attention due to their capacity to operate underwater and in the air. However, the significant impact load during high-speed water entry poses a great challenge for the cross-medium flight vehicle design. To alleviate the extreme impact load, a novel parabolic configuration for cross-medium multirotors is proposed in this paper and parameterized via cabin bottom curve shape and model length. The Structured Arbitrary Lagrange-Eulerian (S-ALE) method is employed to analyze the characteristics of impact loads during high-speed water entry. An optimization problem for the multirotor configuration is then formulated to minimize the impact load, considering several constraints, i.e., the volume of the payload cabin, bottom curve height, and minimum curvature. The Kriging-assisted Constrained Differential Evolution (KRG-CDE) algorithm is utilized to improve the optimization efficiency. The optimization results demonstrate that compared to the cylindrical and initial parabolic configurations, the maximum impact load of the optimized configuration is reduced by 20.78% and 9.46%, respectively. This study validates the effectiveness and practicality of the proposed cross-medium multirotor configuration and optimization framework in reducing impact loads upon water entry.