This achievement is pretty simple. When you get to the level 'sundown' you will hear Simon (the man that helped you out of Venice) say to find the church and to "stay out of the water if you want to live." It should give you a checkpoint straight away, and if you run forward a bit you will see a small pool of water. Dive into the water and Artyom will look around and jump out. You should see a disc spinning in the bottom left of your HUD. After you regain control, dive into the water again. This time a shrimp in the water should kill you, making you reload checkpoint. Simply repeat this process four more times and the achievement will pop!
Every time you dive into the water you should see the disc spinning, indicating it counts towards the achievement.
Note: After the second "dive" feel free to reload the checkpoint in the pause menu once the autosave symbol disappears, it saves a bit of time.