diff --git a/Tools/Scripts/MapGenerator.cs b/Tools/Scripts/MapGenerator.cs
index 66b6991..f44835d 100644
--- a/Tools/Scripts/MapGenerator.cs
+++ b/Tools/Scripts/MapGenerator.cs
@@ -545,7 +545,15 @@ public partial class MapGenerator : TextureRect
 				if (_coastWide && finalH < seaHere)
 				{
 					float depthM = (seaHere - finalH) * 251f;
-					finalH = seaHere - IslandFalloff.CoastShelf(depthM) / 251f;
+					// The remap is strictly positive on positive depth, so in exact
+					// arithmetic the waterline cannot move. In float32 it can: for a
+					// pixel a few microns under water, (sea - shallowerDepth) rounds
+					// back up to exactly sea, and `H < sea` then calls it land. That
+					// cost 5 px of 67 M on the first batch — immaterial in itself, but
+					// it falsifies the invariant the whole separability argument rests
+					// on. Hold the result strictly below sea and the invariant is exact.
+					finalH = Mathf.Min(seaHere - IslandFalloff.CoastShelf(depthM) / 251f,
+						System.MathF.BitDecrement(seaHere));
 				}
 
 				// --- 6. OFFSHORE ISLANDS (task 11) ---