Dynamic behavior of the underwater smart elastic structure has gained considerable attentions, and been widely used in the applications of precision instrument, bionic robot, autonomous underwater vehicle, and so on. A electricity-structure-fluid coupled model of a elastic cantilever with partially bonded MFC actuators is proposed, and the frequency response experiments of the underwater structure with different excitation levels of the MFC are carried out. The equivalent structural and dielectric parameters of the representative volume element of the MFC actuator are obtained using the mixing rules method. Based on the assumed mode method, the segmented first-order normalized mode shapes of the cantilever with partially bonded MFC are determined. It shows that the displacement at the end of the active deformation section with bonded MFC actuators is only 3% of that at the tip of the passive deformation section, and the mode shapes of the proposed cantilever changes significantly, compared with the cantilever with a uniform cross-section. Then, the electricity-structure-fluid coupled dynamic equation of the proposed structure is established, the internal actuation moment of the actuators, the hydrodynamic load provided by the surrounding fluids and the vibration behavior of the elastic structure are all considered in the coupled model. Using the established set-up, the frequency responses of the underwater elastic structure at different excitation frequencies of the MFC actuators are conducted. Experimental results show that the magnitude-frequency and phase-frequency spectrums of the underwater elastic structure in experiments match well with those of the established model. The effectiveness and feasibility of the obtained coupled model are demonstrated.