The Optimal Path As Raster tool can be used to 'trace downstream' using the same flow direction raster used to make watersheds.
The optimal path created can be a flow path based on D8 flow direction. To generate an optimal path in this way, use a D8 flow direction raster as input for the Input back direction or flow direction raster parameter. You also need to supply an Input distance accumulation raster value; the Input distance accumulation raster value is not used to determine the path. Whether you use a constant raster or a digital elevation model (DEM), the path will be the same; only an attribute value on the path will vary. See the Flow Direction tool documentation for more information on D8 flow direction rasters.