What do the following lines mean The water-wraith was shrieking And in the scowl of heaven each face Grew dark as they were speaking?

The storm suddenly grew very fierce, the water aas if had spirit was shrieking, the atmosphere as very intense, everything was gloomy. A scowl on a persons face indicates anger. The poet uses this phrase to elaborate the darkening of the skies and the thickening of the clouds. This increases the intensity of the poem. The scowl or the heavens actually seems to foretell that some calamity is about to befall on the lovers.