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It doesn't need to be in one go; you can do it in multiple zones. Whenever you find a body of water deep enough to submerge, go sit underwater until Blake starts to drown, go out, and then go back. If you can't sit through it until you get the achievement, just sit underwater until Blake starts to drown, go out, and then in, a couple of times, then progress with the story until you find another body of water, and then do the same. I got it in Chapter 3, The Lake; I submerged in water for a couple of seconds, and then I got the achievement.