The effects of fluid‐structure interaction on the force transmissibility of mechanical systems elastically mounted in a submerged elastic shell are investigated. An analytical solution for the ease of a harmonic‐force‐excited two‐degrees‐of‐freedom undamped system attached inside a spherical elastic shell is obtained by the separation of variable technique. The transmissibility of the internal system is computed for a broad range of excitation frequencies and compared with that of the same system mounted on a fixed base. Due to the loss of energy through the radiation of the elastic shell, the transmissibility resonance peaks of the undamped system elastically mounted in the submerged shell are reduced to finite values and, for other nonresonance frequencies, the magnitude is in general also reduced. Moreover, the fluid‐structure interaction effect shifts the peaks of the transmissibility towards lower frequencies. This information could be useful for the sound isolation of systems inside a submerged vessel.