These lines represent both metaphorical birth and perhaps the literal act of being caught as a fish. The woman is pulled from the water and into the light, perhaps even by Wandering Aengus of Irish lore, and is thus reborn as a woman. The light signals an awakening of her inner natural self, which, despite her utter transformation, cannot be totally eradicated.
In W.B. Yeats' "Song of the Wandering Aengus", the young man catches a fish who turns into a "glimmering girl" and runs from him. This girl is very beautiful, but the fact that the man searches for her for an eternity labels her as more an object to be sought after (one who wishes to be chased) than a woman capable of independent thought and action. (read "Song of the Wandering Aengus" and other Yeats' poems at Connections: Other Poems by Yeats).