As Jill progress through her mission until you reach the huge multi-storied gold room (hall). Head all the way to the top floor to find a sign near a door saying "Solarium". As you enter the area, head around the side of the pool (which Jill will NOT jump into, cause she doesn't want to get stinky). You'll find a circuit puzzle on a nearby door that you'll need to complete to access the pool pump room. Turn the pump on to allow you to swim in the pool in a later mission (you can come back in Episode 4 or 5).
Note: This is also required to allow you to access a hand print near the diving board, so if you got this achievement on one run, and are going for the hand prints on another run, you'll want to repeat this process both times.
This one is easily done by going to the Solarium as Jill early on and filtering the water from the pool, then you will come back later on as another character and be able to go into the pool, which you would not be able to if you had not started the water filtration as Jill as soon as you enter the big room with 3 floors, with the Solarium on the 3rd using the stairs as the elevators are unavailable. If you here some creep mad voice calling for radio distress... simply back track.