Quadrotors have been used in diverse areas, such as agriculture, civil construction, fire monitoring, and so on. However, these vehicles cannot perform missions that require navigation in aquatic environments, such as inspection of underwater structures and water sample collection, due to their hardware limitations. Therefore, hybrid unmanned aerial-underwater vehicles emerge as a feasible solution to accomplish these missions. Although quadrotors have well-accepted mathematical models, hybrid vehicles have a small quantity of works dealing with this topic. Moreover, we identified a gap in transition zone representation. Therefore, this paper presents a method for transition zone modeling based on vehicle geometry. Simulation results support the better representation of the proposed transition model in comparison to a sigmoid model.