The kettle is located on a high platform directly to the left of the entrance to Area 3. Various obstacles including climbing up walls have to be passed to reach it, however.
From the starting point, carefully head over to what appears to be a large gap with a Balloon Fish placed directly in the middle.
You will then receive a message from Cap'n Cuttlefish informing you that the Octarian forces have hidden the path. Shoot the Balloon Fish to reveal the hidden paths. As soon as you have made it over, head over a small ramp, and you will notice a relatively small gap with two Octocopters on the other side. Hidden beside the gap are several invisible walls which you can use to your advantage. As soon as you move in closer to take out the Octocopters, Cap'n Cuttlefish will once again message informing you of an Octoling ambush. Deploy a Splat Bomb or use your Hero Shot to take out the Octocopters and then move on to your next target: the lone Octoling. Once the enemies have been dispatched, a launchpad to Checkpoint 1 will be revealed.
After you have Super Jumped from the beginning area, there will be an Octopod. Splat it, then move forward, where there is an Octostamp. From there, if you move forward, there will be two Twintacle Octotroopers and a barrier to hide behind. Once you have splatted them, head up the invisible ramp. Turn around and you'll find an Octobomber. Once you have splatted that as well, head to the next checkpoint.
After you reach the second checkpoint, there will be some Octoballs, rolling down on an invisible ramp. Before they roll down, however, there is a crate containing the Sunken Scroll that gets covered up by the ramp when it is inked. When you get to the top, ink the right side of the ramp to reveal a small invisible platform. Jump down onto it then go under the ramp. Get the Sunken Scroll inside the crate, then go back to the invisible platform. Ink a path up an invisible wall to get back to the top of the ramp. There will be a checkpoint.
You will Super Jump to Checkpoint 3. There is a wall and only one hole to get to the other side. Find the hole, swim up and into it, and on the other side, there are three Twintacle Octotroopers there. Go and splat them. There are two crates with Power Eggs in them. Get those, then go up the wall, and Super Jump to Checkpoint 4.
When you get to Checkpoint 4, you will see some Power Eggs. Get them, and that will reveal the hidden path. The path is narrow, so moving while a squid is not a good idea. Once you turn the second corner, an Octocopter will appear. Splat it, then proceed. Ink an invisible wall, then you will encounter a Squee-G. Ink another invisible wall, then proceed on the path. The path is a tiny bit more narrow. Once you pass the third turn, another Octocopter will appear. Splat it, and proceed again. Another Squee-G is on a wall. Pass it, then find the path, which is even more narrow. Once you get halfway across the first strip of the path, an Octobomber will appear. Splat it quickly. The path ends after two more turns, where there is a launchpad.
From Checkpoint 5, go down, and you will see some walls. Approach the first wall and some Octocopters will appear in the distance. Behind the first wall is a Twintacle Octodiver. Splat it, then focus on the Octocopters. There are two Balloon Fish to ink a wall and maybe splat some of the Octocopters. Splat the rest and go up the wall. The final checkpoint will be up the wall.
Break some crates and you will receive some armor. Go past a wall and a Twintacle Octotrooper will appear on an invisible platform. Splat it, then proceed. Climb onto the platform and you will see two Twintacle Octotroopers on two separate platforms. Splat them and proceed again. More invisible ground will appear, then three Twintacle Octotroopers will appear. Do your best to splat them. Another set of three Twintacle Octotroopers will appear, but farther away. Splat them then proceed to the Zapfish.