Instructions
Start the Secret Agent mission called "Clockwork Repairs" by clicking on the "Top Secret" icon on the lower, right corner of the screen after completing the first six missions. Click on Gary to talk to him and read through your mission parameters.
Click on the arrow on the right side of the screen to scroll to the right after your conversation is over. Scroll all the way to the right and you'll see several gadgets lined up on shelves.
Click on the swimming tube on the right side of the top shelf to pick it up. Hold down the left mouse button to drag the swimming tube to the inventory tab at the bottom of the screen, then release the button to place it in your inventory.