In recent years, increasing attention and expanding research have been devoted to the study and application of soft actuators. Inherent compliance equips soft actuators with such advantages as incomparable flexibility, good environmental adaptability, safe interaction with the environment, etc. However, the highly nonlinear also bring challenges to modeling of dynamics. This study aims to explore the dynamical characteristics of an underwater hydraulic soft actuator. The actuator has three fiber-reinforced elastomer chambers distributed symmetrically inside. By controlling the pressure in the chambers through a hydraulic power system, the actuator can achieve spatial motion with three degrees of freedom. To describe the relationship between the input pressure combination and the actuator movement, a dynamic model considering the nonlinearity of viscoelastic material is developed based on Lagrangian method and constant curvature hypothesis. A series of experiments are carried out, including single-chamber actuation and multi-chamber actuation. The test results verify the effectiveness and precision of the model. Finally, the effects of the geometrical features on dynamic response are investigated through model-based simulation, which can provide guidance to parameter optimization. The proposed dynamic model can also contribute to behavior analysis, performance prediction, and motion control of the hydraulic soft actuator.