The presence of substructures in a cylindrical shell gives rise to dynamic interactions which influence the distribution of vibration along the shell. In this paper, Lagrange's equations are used to develop the equations governing the forced harmonic vibration of a submerged cylindrical shell with periodically attached internal substructures which is subjected to periodically placed loads. Modal expansions are used to describe the tesponse of the shell and the fields in the fluid. First, a shell without substructures but subjected to periodically placed ring loads is considered. The method of this paper gives nearly the same results as a travelling wave method for an infinite shell subjected to a single ring load. Two types of substructures are examined: circular panels with hinge connections and circular ribs with rigid connections. It is shown that the substructures confine the flexural motion to a region near the location of the applied forces. The localization effect decreases when the forcing frequency is close to the substructure's natural frequencies, and becomes more pronounced when, within a periodic segment, the ribs have varying impedance.