How to Supervise in a lifeguard chair.

You need to level up and do the story missions to the point you can build the beach, then build it and then bring a Sim there to complete the task.

Where the beach. I see only the Swimming center.

It is the toy-like thing to the left of the entrance

So basically when it says have a sim supervise at the pool. Go to the swim center and click on it. It won't work unless you have actually reach the task

Good luck xx

There is a lifeguard chair in the Swim Center in town (assuming you have built it) and there are Life Guard Chairs at the Beach - if you have built that as well...

The Lifeguard Chair is found on the beach, on the Mystery Island. So once that is unlocked AND you have completed the Senior Sims / Aging Missions, you can build the Beach and thus have access to the chairs - OR you can see if a neighbor has one you can use.

Click the chair in the swim center in the town-if you have completed the Diving Hobby twice you can put a life guard chair in your house-I have one and if you still need it respond back and I can tell you how to become a friend on The Party Boat so you can use mine at my house.