Erik Kostson
Ansys Employee

There is a big specific impedance mismatch between air and water (since it is impedance~rho-c_speedofsound), so the acoustic waves from/in the water will be reflected there at the interface (watre/air) since the reflection coefficient is close to 1, and the same if the waves come from the air into water.
So the water is like a waveguide and the acoustic energy stays inside there .

Now the boundary condition (BC). So i think a zero acoustic pressure (p=0), so sound soft BC, should be OK there at the air/water interface (otherwise we can model the air and the water as acoustic media).
All the best