fix: polygons уже содержит polygon.coordinates — MultiPolygon.coordinates = polygons

backend/routers/water.py

@@ -177,10 +177,10 @@ def _water_geoms(elements: list[dict]) -> list[dict]:
             if len(polygons) == 1:
                 geom = polygons[0]  # {'type': 'Polygon', 'coordinates': [ring]}
             else:
-                # GeoJSON MultiPolygon: собираем полигоны в один Multi.
+                # GeoJSON MultiPolygon: coordinates = список polygon.coordinates.
                 geom = {
                     'type': 'MultiPolygon',
-                    'coordinates': [p['coordinates'] for p in polygons],
+                    'coordinates': polygons,
                 }
             out.append({'layer': layer, 'name': name, 'geom': geom,
                         'area_m2': sum(areas),