The Zoroastrian goddess of waters and fertility is Anahita.
Her full Avestan name is Arədvī Sūrā Anāhitā, commonly rendered as Aredvi Sura Anahita. She is the divinity of the waters and is associated with fertility, healing, wisdom, purification, and the prosperity of people, animals, and crops. In Zoroastrian tradition, she is a yazata, a divine being worthy of worship, rather than the supreme creator deity.
Anahita is especially prominent in the Aban Yasht, where she is portrayed as both a powerful goddess and the source or embodiment of the world’s flowing waters. Later Iranian traditions and neighboring cultures sometimes identified her with goddesses such as Aphrodite, Artemis, or the Indian Saraswati. Mithra is associated chiefly with covenants and light, while Tishtrya is linked with rain and the star Sirius; neither is the canonical answer to this question.