For coupled simulations of waves propagating along a fluid-solid interface, the homogeneous Dirichlet condition is the physically correct condition to describe the "free-surface condition" in water.
Homogeneous Dirichlet conditions force the wavefield to be zero at the specified boundary.
In the scalar acoustic case, this simply gives
and in elastic media, we obtain
It is possible to fix only certain components of the displacement field in the elastic case.
More information on the practical use of absorbing boundaries can be found on the following pages: