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This stage starts at the entrance of the Aquatic Fun Center. Take the coupon from Scarlet, walk between the line dividers, and stand behind the line of people until it's your turn at the ticket booth. This display of patience unlocks an achievement/trophy. Place the coupon on the counter and take the ticket at your turn. Before returning to Scarlet, use the nearby ladder to climb on top of the ticket booth. From here, climb onto the protruding segment of the wall to the left and follow it across a narrow bridge to the other wall, where you should see a tie (1/3). You can drop the ticket down below if you need a free hand to grab the tie, as the turnstile should be right underneath this area.
Before passing through the turnstile, you can enter the men's room to the right. Stand in the toilet in the center stall on the left and flush to access a secret level: Toilet and Trouble. Otherwise, you can continue past the turnstile to proceed through the stage as usual. If your ticket is nowhere to be found after having completed the secret level, simply return to the ticket booth to pick it back up, though you'll have to avoid the eyes of the marine biologists by cutting through the gift shop to do so. Past the turnstile, move behind the cardboard cutouts to avoid the marine biologist and proceed up the stairs.
You are now in the hub of the Aquatic Fun Center, where the subsequent stages can be accessed. Drop down to the right to find a vending machine, which you can reach inside to grab a tie (2/3). You cannot get the third tie yet, so enter the World of Kelp to begin the next stage.