When the gods left Yastr not everyone was ready for a life without religion. While some turned to almost religious cults of their leaders, others started to worship the Nereids. Nereids are small humanoid creatures living in all kinds of bodies of water, lakes, rivers and sea.
Lords of the waters, grant us with good harvest this year! We are thankful for your protection!
Believed to be immortal or, at least, very long lived
I'm aware that in Greek Mythology nereids are nymphs of the sea. Yastr is an alternate world, so I don't see anything wrong with its nereids being associated with every type of water.