With the increasing demand for the exploration of marine resources and the observation of marine issues, many aquatic-aerial vehicles were developed. This paper developed a small agile water aquatic-aerial vehicle that can fly in the air and swim in water. A foldable mechanism was proposed to reduce excessive underwater resistance caused by its specified flight structure, and a bi-stable structure was proposed to maintain the stability of the foldable mechanism and reduce the consumption of the servo. The mechanical modeling of the foldable mechanism is established. And the servo with the torque of 45kg$\cdot$cm is able to open the foldable mechanism 0.4s to complete the mode conversion of cross medium when the aircraft jumps out of the water. CFD simulation was also carried out in water. Driven by thruster with 38N propulsion, the vehicle reaches a speed of 5.1m/s. Finally, the vehicle prototype was assembled. In the future, we will focus on underwater motion and aerial flight experiments, and ultimately achieve cross medium flight.