The dynamic characteristics of a structure, which is partially or totally submerged in a fluid medium are substantially different from those due to its vibration in absence of a medium; the fluid in the Civil Engineering context is usually water, which con- siderably modifies the response of the structure to a given motion, the extent of modi- fication depending on the characteristics of the structure and the fluid body. A very important aspect of the problem is the interaction between the structure and the fluid; the vibrating structure generates Hydrodynamic pressures which modify its deformations; these deformations in turn modify the Hydrodynamic pressures causing them. The problem is thus recognised as a “coupled” or Elasto-Hydrodynamic one, in which the structure-fluid system may be considered to respond to excitation as a unit, having its own ‘coupled’ dynamic characteristics, which are different from the uncoupled characteristics of the structure and the fluid separetely. Exactly how different the coupled characteristics would be compared to the uncoupled ones, would depend on the so called “coupling factor” (8) between the structural and fluid phases. For example, for a flexible structure, the effect of coupling would be only to the extent of its so called “added mass” (11, 12), while, for a stiff structure, the characteristics would not be very different from those of the fluid-body itself.