Abstract: Aerial-aquatic rotorcraft (AARs) have garnered extensive attention due to their ability to operate in the air and underwater. However, their widespread application still faces challenges, the most prominent being the stable transition across water surfaces. Neglecting the near-water-surface effects during the transition process may lead to distortion in its system dynamics model, increase the difficulty of controller design, and even result in failure during the transitions. Most previous studies have relied on the ground effect theory to simulate the transition across water surfaces, but these efforts have shown limited accuracy in predicting rotor lift due to the significant differences between ground and water. In the present paper, we investigated the rotor's aerodynamics in proximity to the water surface using the potential flow theory, based on which a rotor aerodynamic model incorporating the near-water-surface interference is proposed and optimized by Laplace's law. Experiments conducted at various heights above the water surface and with different throttle settings have validated the superior predictive capability of the proposed model. The results indicate that the lift increases as the rotor approaches the water surface, but the increment is less significant than when it approaches the ground. Additionally, the lift increment induced by the near-water-surface effects diminishes at higher rotation speeds. Moreover, the aerodynamic model of the rotor and the correction method adopted in this study significantly improve the prediction accuracy of rotor lift under both near-ground and near-water-surface conditions.