Erik Kostson
Ansys Employee

Please see and look in detail at the posts I have posted a link for.
In particular see this post above, and to answer your question (1), we only need a modal acoustic system (so no static or system coupling).
In the modal acoustic system we need to have both the structure (steel structure) and the acoustic domain (water) in the same model/geometry, and they should share topology because we need a compatible and fully connected mesh between the structure and the water (so contacts are not recommended as they can cause issue in this type of analysis). Have in mind also that we can not account for any velocity or movement, we will just calculate the wet modes of the structure in water, so we are not doing a hydrodynamic analysis to see the motion as it moves through the sea. The above modal acoustic calculates the wet modes of the ship so first bending mode, etc. If you need the ship motion use Aqwa, hope that is clear now.

Finally to calculate the wet modes (ship) in water, it does not need any boundary condition since it is a free un restrained structure in water.

I will not be able to help anymore on this, but as we discussed the basic principle is outlined above and in the lionk I suggested - for any details perhaps other forum users might be able to provide some help.