To get the water tunic you have to find or buy it.
You never kill Zelda.
You need to get the Longshot, the main treasure in the Water Temple in Lake Hylia
The Zora's Tunic (blue tunic) can make you breathe underwater so you don't have a time limit. The tunic comes in very handy in the Water Temple
yes, before you go too the fire and water temple you will be given a blue and red tunic
you need to get the zora tunic
If you're refering to Twillight Princess, you can't swim underwater while in you're wolf form. You can swim underwater as a human while wearing the Zora Tunic.
Breathe under water
you need water bombs, the blue tunic, and iron boots. go to the bottom of lake hylia with the tunic and boots and you'll see it. not sure if there is a mechanism to open the door but it shouldn't be too difficult.
You should try a walkthrough
Go really fast.
I dont think you can. Unless there is a cheat somewhere.
you must lower the water