ὕδασι

From LSJ

φέρουσα κατακρύπτει ἐς τὸ ἀφραστότατόν οἱ ἐφαίνετο εἶναιwherefore she bore it away and hid it where she thought it would be hardest to find

Source

French (Bailly abrégé)

dat. pl. de ὕδωρ.

Russian (Dvoretsky)

ὕδασι: dat. pl. к ὕδωρ.