Quick answer: Unreal Water Body Custom Mesh slicing into a shoreline mesh, producing a sharp visible edge? Single Layer Water shading + a feathered water mask blend the seam.
A river meets a sandy shore. The water mesh ends in a hard line; the shore mesh shows through. Players see a paper-thin boundary.
Single Layer Water
Material shading model = Single Layer Water. The water surface receives shoreline geometry data and shades against it — produces wet sand, foam at edges, proper transmission.
Water Mask Texture
Authored grayscale texture controlling where the water surface exists. Feathered edges blend the boundary; sharp edges produce sharp transitions. Paint per-river / per-pool.
Foam at Shore
Sample the shoreline distance from the water depth buffer and add foam where depth approaches zero. Cheap shoreline effect that hides imperfect mesh alignment.
Z-Fighting
If the water mesh is exactly coplanar with terrain near shore, raise the water mesh by 1cm to avoid z-fighting. Single Layer Water tolerates small overlaps gracefully.
Verifying
Shoreline shows a soft blend instead of a hard line. Wet sand visible where appropriate. Foam at the meeting line during animation.
“Single Layer Water + a feathered mask hides shore seams.”
For inland water (ponds, lakes), authoring the mask in Substance / Photoshop gives you per-place control — way cleaner than world-machine-style procedural masks.