im wondering if theres a way to make coastal undercuts like shown below directly next to water and also on beaches
Not inside world machine, or any other “heightfield based” terrain modelling app, no.
All effects in WM (and by extension, all terrain modelling apps) go in one direction, UP (z-axis, perpendicular to the horizontal xy-plane).
You can make any such overhangs (caves, cliff faces, coastal undercuts, etc.) using another 3D sculpting app like zbrush, or Blender.
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