Zora's Domain

In Zora's Domain, go back up the hill and show King Zora the message. He'll
then move over, letting you get into Zora's Fountain and Jabu-Jabu. Go down
to the shallow sections of the Domain and catch a fish in your Bottle, then
go behind King Zora and head to Zora's Fountain, home of Jabu-Jabu!