Erik Kostson
Ansys Employee

First of all I would like to recommend our courses in modal analysis :
As for the second question. Modal analysis is a linear analysis so we can not include nonlinearities (large deflections, contacts (frictional), and material nonlinearities). We can ofcourse include large deflections (geom. nonlin.) in a static analysis (and of course plasticity and frictional contacts), but not in modal.

Third one. For pre-stress modal analysis see our help manual (search for modal analysis and prestress). A pre-stress generates stress stiffening or softening effect on the structure that is it (so as we tension a guitar string the bending modes/frequencies increases, so stress stiffening effect, that is all we can do, and include in a prestress modal)

For your first question: for added mass effect on the vibration of submerged structure (one can not have beam elements - only 3D structural bodies and hence 3D elements are allowed) search this forum, there are quite a few threads (there is no demo which I know of):
All the best of luck