You can get this achievement as soon as you get off the elevator leading into the Lambda Main Reactor. When you step off of the elevator, you will see a sign that says Lambda reactor complex. In front of it is an open area that is fenced off. Down below, you will see bluish water. This is the coolant that you will need to drown in. You can either jump from the railing, or descend the ramp ahead of you and to the left to make your way down lower. All you have to do is just jump in the water and drown yourself (i.e. until your health completely dissipates).