The waters are all the Devas. Waters are the Pitris. The Devas have equal rights with the Pitris to perform the sacrifices, called Darsha and Paurnamasa. Darsha and Paurnamasa yajnas are performed for both Devas and Pitris.
आपो देवगणाः सर्वे आपः पितृगणास्तथा। दर्शश्च पौर्णमासश्च देवानां पितृभिः सह॥