SAY-loon
Selune's presence is mainly felt in the upper level of the ocean, though the acknowledgement of the pull of the tides by the moon is felt through levels.
The story of what attacked Selune is lost so the waves of time, scoured clean like the bed of the sea after a storm. But the fact remains: the goddess was attacked, her home damaged, and part of it fell to Errbeolaus below. Those pieces of her home and person sunk deep into the sea, and at first there was conflict, for the goddess had no place in the ocean and longed to protect and reunite with her lost self.
It was not to be so, but in lieu of that a compromise and new contract was forged. Dionadier took up the protection of the moon, and so her temples are keepers of the Moonstones, pieces of Selune that have sunk into the ocean and reflect the face of the moon above, shining even so below. These wax and wane with the tides, and have over the centuries begun to influence the ocean life around them. And it is under their light that Dionadier blesses and protects her followers.
Details borrowed from: https://forgottenrealms.fandom.com/wiki/Sel%C3%BBne