I have not used the Ansys Workbench, but, being somewhat versed in elastic & acoustic wave modeling and underwater acoustics, and by looking at your diagram, it seems to me that you need to put reflective sea-surface boundary conditions into the model.

Presumably the sea surface is at the upper horizontal boundary in your diagram (the waterline for the ship hull).

The sea-surface is essentially a pressure-release boundary for acoustic waves; that is:

Pressure-release boundary conditions are approximate. Some small underwater sound energy does cross into air. But the pressure-release approximation is very good owing to the very high impedance mismatch between water and air. Indeed, pressure-release conditions are used at the sea surface in virtually all of underwater acoustic modeling (sonar, seismic, marine-impact, etc). The only exception that comes to mind is if you were expressly interested in modelling the tiny fraction of sound energy that passes from underwater into air (to estimate how loud a deep underwater blast would sound to people above the water for instance). I'm sure that the pressure-release boundary conditions at the sea surface serves perfectly well in your case.

Ronald

Thanks for your answer Ronald.

I found a way for my purpose, you can give the "Acoustic Far Field Microphone" in Ansys the boundary contidtion, to make a sound soft boundary on a plane (from a coordinate system). So on the surface, of the acoustic soft boundery the pressure is zero, like it should be. 

Conrad

Does it change your figure notably then? It would be interesting to see its effect.

R