You can get this achievement fairly early in the game (either the 4th or 5th chapter, depending on the order you choose). It's the chapter called "The Confession" where you play as Lucas when you go to meet your brother in the park (the first Lucas chapter after you leave your apartment for the first time).
After the conversation with your brother you have a vision of a child drowning and then you see him fall in the lake. Head toward him and you'll automatically jump in the water. Play a mini-game to swim down to him. Perform an action to grab him. Perform three more mini-games to swim back up, to put him on the shore, then for you to climb out of the water.
Stand up, approach the boy, then kneel down. Choose CPR. The trick here it to wait until after every 3 count to do a compression. Your character will count "1...2...3" then you compress. Do this correctly a bunch of times and the kid will revive and you'll get the achievement.