When entering Area 3, head right and find two steep ramps. Use the ramps, jump on the platform nearest to you, then jump to the left where there are more platforms. Another ramp can be found, so swim up the wall and reveal the hidden kettle.
When you start, you will first see an Inkrail. Swim through it and you will end up on a small island with two curved ramps and a Twintacle Octotrooper will be waiting for you there. Splat it and then use the Inkrail to get to the next area.
The Inkrail from the beginning area will take you to a rectangular platform with a checkpoint, a new Inkrail, and a Twintacle Octotrooper on top. The original Inkrail wraps around the platform and can be used to stealthily splat the awaiting Twintacle Octotrooper. Once the Twintacle Octotrooper has been defeated, take the new Inkrail.
This section is similar to the first one, except that there will be two Octodivers hiding in the ink. Using the same method as in Sector 1, jump up from the Inkrail and splat them. The Inkrail to Checkpoint 2 is in the middle.
From there, you will be on a platform with another Inkrail. It takes you under another platform and goes through a grate in the middle of it. Jump off it, then splat the two Twintacle Octotroopers riding around it. Then, a launchpad will appear.
After landing, activate the Inkrail and ink the wall above it so you can jump from the rail and climb up. Once on top, splat the two Octopods. You will now see three Inkrails that you need to activate to jump from one to the other to reach the top, but the middle rail goes around the building. Taking it will lead you to the Sunken Scroll. Continue up and take the Launchpad towards the next Checkpoint.
After landing, you now have two stationary Twintacle Octotroopers in your way. Take them out and head up towards the launchpad.
Swim through the Inkrail—you will land in the middle of a circular platform surrounded by water with a 'ring' on the outside. There will be some Octarians on the outside of the platform and two small curved platforms with Inkzookas on top of them. Take them. You can either use the Inkzooka to splat the Octarians from the middle or ink the sides of the ring with the Octarians, then swim, jump and climb up it so that you are on the same platform as the Octarians. Once all of the Octotroopers have been splatted, a launchpad will appear to the final checkpoint in the middle. If you jumped to the outer ring, you will have to paint the walls to jump back to the middle.
Swim through the Inkrail that is in front of you to get to a large area with three towers. The Inkrail will take you to the first one. Jump off it then splat the Octotrooper on it. Repeat for the next tower. On the third tower, a Twintacle Octotrooper will be waiting for you. Splat it and then swim up onto the final tower, which will have the Zapfish. Burst its shield, then take it.