Once you free king zora he says "Oh my dear preinses ruto where has she..

First go to Lake Hylia and find the Bottle with Ruto's message inside (near the shore by the Scarecrows), then show the King the Message and (5 minutes later) he will have moved out of the way. Put a Fish in your new Bottle from the water below and then go behind the King to the next room. Now lay the Fish in front of this Whale named Lord Jabu Jabu...

You said after you freee him right? So that means your near the Water Temple. There's a simple answer! Princess Ruto is in the Water Temple.To get into the Water Temple go to Lake Hylia using the Serenade of Water get the iron boots on and go to only wet part of the lake. You'll see a gate in front of you. To get in get your hookshot out and fire it at the blue diamond thing right above the gate. The gate will open. (Be sure to wear your zora tunic while doing this or you'll drown!) You'll want to wear your zora tunic during the whole of the Water Temple but sometimes you'll have to switch from iron boots to Kokiri boots. Once you get in the Water Temple take the iron boots off and you'll float upwards a little bit. Swim straight. Put the iron boots back on and you'll go to the very bottom of the temple Once your at the bottom you'll see a lot of openings to different rooms. Princess Ruto is in one of those rooms. When you find her she'll talk about the long seven years you made her wait for you and blah blah blah. After she's done talking she'll swim upward. Take off the Kokiri boots and follow her at the top you should see an inscription in the wall to the right that looks like three triangles put together. Get infront of it and pull out your ocarina. Play Zelda's Lulaby and the water level will lower. Don't worry if you don't see the princess at the top you don't really need her. After that you're on your own.
Hope I helped!