A modification of traditional FEM/BEM called element grouping method (EGM) is proposed to calculate the elastic vibration and acoustic field radiated from an underwater structure. Instead of rebuilding the structure using coarse FEs to save the computational time and data storage, the structural FEs on the coupled surface are grouped into clusters of BEs to reduce the number of fluid DOF. With the grouped elements (GEs) used as BEs, the problem of non-matching between fluid DOFs and structural DOFs is solved. The approach is illustrated and validated using known analytic solutions for submerged spherical shells subjected to uniformly applied incident pressure. Acoustic radiated from a ribbed cylindrical shell driven by a force is predicted by using different element grouping styles. Comparison between numerical and experimental results shows that fine grouped elements give a high computation efficiency and more efficient data storage saving.