Water polygons are simplified too much. You can see that in the style from [map.geofabrik.de](http://tools.geofabrik.de/map/#11/48.9850/8.4476&type=Geofabrik_Basic_Colour) at zoom level 11.0:  And if the style is created with Maputnik, zoom 11 includes the range from 11 <= zoom < 12. The screenshot is at zoom 11.61: 