The Legend of Zelda: Breath of the Wild’s "A Wife Washed Away" side quest begins after you calm down the Divine Beast Vah Ruta and return to Zora’s Domain. You might bump into a zora named Fronk. His wife went fishing around the time that Ruta started causing problems and she got swept downriver. It’s up to you to find her.
You have to speak to Fronk in Zora’s Domain to pick up this side quest. At night, he’ll be in the communal sleeping area on the middle level of Zora’s Domain — the level above Ne’ez Yohma shrine. During the day, he’s working on the stonework on one of the platforms on the eastern side of the city. He’s easy to find because his children are running around him.
The long way to complete this quest is to just start following the Zora River downstream until you happen across Mei. The short way to complete this quest is to read the next paragraph.
You’ll find Mei on a island in Lake Hylia just east of Hylia Island way, way, way downstream from Zora’s Domain. The fastest way to get there is to fast travel to Ya Naga shrine. Hop over to the tiny island immediately to the east, and Mei will walk out of the water to greet you.
It seems she just got carried away fishing and completely forgot about her family back home. We’ve all been there. She’ll give you five staminoka basses when she leaves.