Locate the gadget inside the McMarx Sports Shack is a mission in Ratchet & Clank. Clank recommends that Ratchet investigate the McMarx Sports Shack to search for more items. After doing so, they obtained the Hydrodisplacer.
This mission must be completed to complete "Search the sewers" on Rilgar, as the Hydrodisplacer is required.
Walkthrough[]
Enter the McMarx Sports Shack using the same path taken in "Find Skidd's Agent". After taking the elevator to the Sports Shack, take the path on the left. Hold 
to use the Swingshot to pull towards the versa-target, and then swing across the next versa-target to reach the platform on the end. Here, you must use the Swingshot to traverse several versa-targets, using it to pull towards them and reach different platforms and conveyor belts. Dropping to the platforms below allows you to destroy the crates for bolts before moving on.
Eventually, you will reach a large, round room, with versa-targets across to the end. Use the swingshot and versa-targets again to get to the other side, then to the second platform in the corner on the right, followed by a higher third, also to the right. From here, swing through the room again on higher ground to reach the exit, leading to a final room with a platform surrounded by water. Pick up the Hydrodisplacer here, which can be connected it to the adjacent plug point to drain the surrounding water, then drop down to a bolt crank. Before opening the door outwards with the bolt crank, first swim down the remaining water using 
to reach the gold bolt below, then swim back up, turn the bolt crank, and leave through the door. Here, another plug is found. Plug the Hydrodisplacer in to refill this water, then swim across to the end and take the elevator down.
If the remaining missions "Help Skidd get to his ship" and/or "Find Skidd's Agent" are not complete, finish them before moving on to the next planet. If they are, with the Hydrodisplacer unlocked, you can fully complete Rilgar.