Hello JxBx,
You may try *DATABASE_TRACER and define dummy (floating) nodes at the interface of air and water. Note this interface tracking may not work perfectly because it simply allow the tracers to ride along with the mat vel. At the interface there are 2 materials & this mat mixing makes it hard to track the vel. Sometimes a tracer may get left behind. If you define a few to test you will see how it works.
Ian Do
You are navigating away from the AIS Discovery experience