East Reservoir Lake
Go right and pick up the Knight's Shield in the chest, then go to Sidon at the end of the path who explains you what to do now. Once on his back, you have to break the ice blocks, while Sidon is trying to get closer. To do so use the Cryonis Rune, then when he's close enough, ascend one of the waterfalls on the creature, then quickly, aim and shoot a Shock Arrow at one of the pink orbs. You then have to repeat these steps to destroy the other pink orbs, but the ice blocks will sometimes follow you and become more numerous and dangerous. When you've succeeded, the creature calms down and gets a little out of the water. Sidon leaves you on a platform with a Sheikah terminal, which generates a travel gate. Then you hear Zelda's voice who tells you to look for the Guidance Stone which contains the map.
Vah Ruta, Divine Beast of Water
Go ahead, shoot a basic arrow at the eye-enemy so the pink substance vanishes, then destroy the Guardian further. Look at the gears on the wall, use the Magnesis Rune and spin the gear clockwise until you get the terminal out of the water. Place the Sheikah Slate on it, then Zelda says there are four more terminals. Go near the bars behind you and spot another eye-enemy in the water. Throw a bomb at it and make it explode or fire another arrow. On the other side there is a chest at the bottom of the water. Take it out with the Magnesis Rune and pick up an Ancient Spring inside. Now create an ice pillar under the bars to lift them, go on and place the Sheikah Slate on the terminal to get the Map. Look at it so you understand that you can move the Beast's trump.
Get out, go left, go up the slope and enter on the right. Fight the Guardian, then take its Guardian Sword as well as its Ancient materials. Create an ice pillar into the water, climb it and jump higher to the left. Open the chest which contains 100 rupees, then jump back down and look at the spinning gear with a terminal in it. To reach it, step back and freeze the water sprout's spring when the terminal is at the bottom. Place the Slate on it to activate this new terminal. Get out of the water on the other side, go out, walk up the slope and shoot an arrow at another eye on the ceiling. Get rid of the skull, then keep on going up. Next, move the elephant's trump to the highest position, which causes the water to run on the large gear which starts spinning to the right. While being at the end of the footbridge, use the Stasis Rune on the left-most (external) metallic cube, then when the chest goes in front of you, quickly open it to get an Ancient Core and come back on the footbridge.
Now, move Ruta's trump to the middle, which makes the gear spin in the other direction, and look at the gear's mechanism. When the ball becomes blue, the bars in front of the terminal are lifted. At the end of the footbridge, use Stasis again to freeze the ball when the terminal's cabin is in front of you and the bars are still lifted. Enter the cabin, look at the terminal to activate it, then quickly get out and come back on the footbridge. Look towards the first gear (which should be still), hover to it and turn to the large gear. When the eye-enemy (and the chest) is in view, shoot an arrow at it. Wait for the gear to make another full rotation, then jump on it and open the chest when you can. With 10 Fire Arrows in your pocket, jump on the platform opposite you (trump side), then fight another Guardian. Pick up its Ancient items, as well as its Guardian Spear. Step on the switch, which makes some water flow and creates a shortcut.
Follow the corridor, lower the trump to the maximum, and notice another chest and another eye on the trump. Hover in this direction and shoot an arrow while flying (or after landing) at the eye. Hit the skull that possibly appeared, then open the chest which contains an Ancient Shaft. Move Ruta's trump to the middle to come back into its body, then move it again to the lowest. This time, use the Paraglider to hover to the small platform at the end of the trump (just before the water stream). Next, rise the trump to the 4th slot from the top, and while it's moving, go behind the water stream and go to the terminal. (You can do it in several steps if you prefer.) Activate it, turn around, then with the zoom, notice a chest on a platform to the left. Fly over there, jump above the substance if necessary, get cautiously close to the edge and shoot an arrow at the eye on the opposite platform. Then you can open the chest which contains some Ice Arrows. Now fly on the Beast's head, then go down into the hole. Quickly shoot an arrow at the eye on the ceiling before some skulls show up. Use Magnesis to turn the wheel clockwise twice or three times which opens the ceiling. Then move the trump to the 5th slot from the top so the water falls on the flames below. Go below and activate the 5th terminal. Next, turn around and jump just a level lower near a chest. Open it to get a Knight's Halberd. (If you missed it, you can use the waterfall on the basement and when you're thrown into the air, fly towards the chest on your right). Now you can go back to the starting hall and go down the slope. Look at the main terminal and Waterblight Ganon shows up!
During the first phase, you mostly have to avoid the attacks by doing side jumps or backflips, then as soon as you were able to dodge one, shoot two or three regular arrows into his eye. When he hits the ground, rush at him and deliver as many sword blows as you can. If you manage to dodge with a perfect timing, there are a few slow-mo seconds to hit him (simply hit Y) and cause him a lot of damage. When he has lost over half of its health, the second phase begins.
The water level rises, and you have four platforms to yourself. Quickly swim to one of them, choose the Cryonis Rune and the Shock Arrows. Use Cryonis to break the ice blocks he throws at you, then quickly shoot two Shock Arrows at his eye. When he falls into the water, you can either shoot more Shock Arrows, or get close and strike him. If he aims at you with his red laser, quickly jump into the water behind your platform and you should be protected. Repeat these steps a few more times to defeat him! Don't forget that if you have a hard time beating him, you can leave, cook some food and come back.
Now pick up the well-deserved Heart Container, then get close to the terminal. If you think you've found everything inside the Beast, activate it. Mipha then appears and thanks you for freeing her soul as well as Ruta. She entrusts you with her healing power, Mipha's Grace. Next, Ruta goes under the water to set up south of Zora's Domain on high grounds. The Divine Beast shoots a red laser at Hyrule Castle to help you later during the final battle.
Back in Zora's Domain, go back to the throne room where king Dorephan is awaiting you and thanks you. The Domain is saved. Muzu apologizes to you. The king tells you again about the blade that seals the darkness and leaves a chest for you which contains the Lightscale Trident. Talk to him again to tell him what happened. Then you can talk to the Zoras nearby to start sidequests.