duck

MeshFilter waterMF = water.GetComponent<MeshFilter>();
Mesh waterMesh = waterMF.mesh;

Vector3[] verts = new Vector3[] {
    new Vector3(0, 0, 0),
    new Vector3(1, 0, 0),
    new Vector3(1, 0, 1),
    new Vector3(0, 0, 1)
};

int[] tris = new int[] { 0, 1, 2, 3, 0, 2 };

Vector2[] uv1s = new Vector2[] {
    new Vector3(0, 0),
    new Vector3(1, 0),
    new Vector3(1, 0),
    new Vector3(0, 1)
};


// Assign mesh properties
waterMesh.vertices = verts;
waterMesh.uv = uv1s;
waterMesh.triangles = tris;
waterMesh.RecalculateNormals();
waterMesh.RecalculateBounds();
waterMF.mesh = waterMesh;