First you have to be at your peak in agility, then go to the agency and locate a huge pipe (near the helocopter pad).

Get to where (spoiler alert) you set off the agency beacon and head three levels up on the catwalk. Jump over the rail and make sure you do not hit any of the rock cliffs inside the pipe.

If you did it right you should still be alive and be able to jump out of the water. Then jump off the toilet and look at it, then jump back in and there in the water you see a duck.

Grab it and get to a supply point, then you have the quaker grenade!