On this page of the Disco Elysium guide, you will find a walkthrough for the Close the water lock on Wednesday side quest.
Your goal in the Close the water lock on Wednesday quest in Disco Elysium is to close the sluice to unlock a new location and go to the coast. The task can only be done on Wednesday, on the third day of the investigation.
The task is very simple - wait until the third day, then interact with the control panel at the water lock and pull the lever. After a while, the water lock will close. You can move to a new area - the coast.