After you defeat the impostor Jabu-Jabu, Tri creates an entrance in the rift and you go to the Stilled Jabul Waters.

Explore the Stilled Jabul Waters

Go north and get out of the water. Follow the path and you reach a bridge leading to the west. At the end of the bridge, jump west to get in the sideways water. Go upward in the water until you can walk south onto the highest column. Stack some Old Beds on top of each other in a zig-zag pattern until you can jump up into the water above you. Hold the B button to go to the surface, then go east.

Switch to swordfighter form or use underwater echoes to defeat the dark Tanglers. They drop energy when defeated, so you can switch to swordfighter form after defeating one.

Then go east and there are floating cubes of water. You can press ZR to learn the Water Block echo.

Swim through the first water block, but be careful not to dive too far down while in a water block or you'll fall out. Create water block echoes to the right so you can swim through them and reach the larger body of water on the right. Go to the surface at the right side of this body of water, then go back underwater and create water block echoes to the right so you can swim through to the island on the right.

Get on the bridge and go north. Create water block echoes so you can go up through the water. At the top, go west and defeat the dark blob, then get in the water on the west side and create water blocks to swim west to the next island. Keep going west until you see an island with one square of water in the center. If you need a fairy for your bottle, or just need to heal up, you can use water blocks to go to the island with the square of water, get in the water and dive down until you fall onto the island below, then go south to the island below that. Throw a rock at the wooden box to make a fairy appear.

Then go north and create a water block, then create more water blocks inside of the first block, to make the column of water rise. Do this to reach the next island, then the island with the square of water in the middle. Then make water blocks to the east (start by making a water block on top of the grass) to go back to the island to the east.

Swim through the sideways water, then use water blocks to go past the gap, to where there is a dark Biri floating around. Defeat the dark Biri. Then use water block echoes to go east to the island with sideways boats. Use water blocks to go upward on the boats. Open the chest to get 10 riverhorses.

Go back down the boats, then use water block echoes to go west to the big island there. Go to the left and create several water block echoes in the same place to create a column of water up to the higher water. Get in the water and hold B to surface, until you are in the higher body of water. Go upward and to the left until you can go south to land on the sideways trees.

Go west on the sideways trees until you reach the next body of water. Use water blocks to get close to that body of water, then try putting an underwater monster echo into the water to defeat the dark Tangler, then go farther up and exit on the west ledge, and use another underwater monster echo into the water to fight the dark Biri. Or just enter the body of water and switch to swordfighter form to defeat the monsters.

You can optionally go to the island to the west using water blocks, but keep in mind that at least one of the water blocks in a connected set has to be on solid ground, or the whole set of water blocks will fall. Defeat the dark monster on the west island, then use water blocks to reach the chest. Open it to get 50 rupees. Then go back to the east.

Get on top of this island and go east to see the real Jabu-Jabu, frozen in place. Walk north across him, and you find a building. There will be a cutscene. Go north into the building. You'll be in the Jabul Ruins.