terrain_water_mesh

Builds one node of the water surface from the water / waterMask channels, into the same 23-word vertex layout and the same slot-addressed pool terrain_mesh writes.

The baked column

Alongside altitude, each vertex bakes the WATER COLUMN — how far the surface stands above the bed — into the vertex colour's red channel (word 19): max(water - height, 0) where covered, 0 where not, and a sentinel standing for open ocean on a flat skirt vertex, which has no channel to measure against. terrainWater.shader reads it back as an interpolated attribute instead of reconstructing it from the scene depth buffer, which a plain material draws with no depth copy behind it.

Where a vertex's altitude comes from

the cell under the vertex	y
covered by water	water — the surface itself
not covered	min(water, height - sink)

The uncovered branch is what ends the surface at the shore. height - sink is under the terrain by construction, so a vertex with no water above it is buried whatever the water channel holds there; taking the lesser of the two keeps the plane flat right up to the last covered vertex, so the waterline falls where the plane and the ground cross rather than on a node boundary.

Coverage is read from the nearest cell rather than interpolated: a cell either carries a surface or it does not, and half-covered is not a state the surface has.

The skirt

A node with flat set reads no channel and sits at flatLevel. That is the outer skirt — the ring of nodes carrying the sea from the field's rect out to the horizon, so an island has water all the way around it instead of stopping at the grid. Skirt vertices meet the rect's edge vertices at exactly the sea level, which is what makes the join seamless rather than merely close.

Because the skirt's edge pieces are as long as the field and as deep as the horizon is far, a node here is a rectangle: nodeSizeX and nodeSizeZ are separate.

Stitching

Boundary stitching is the same rule as terrain_mesh: on an edge whose neighbour is one level coarser, the odd boundary vertices snap onto the previous even one. There is no geomorph — a level surface has nothing to pop.