During mission 1, at the first crossroads go up the stairs(not down the submarine). Then at the second crossroads, go down into the boat. Along this route there are a total of 21 POWs you can save, 11 of them very visible: Five on the road to the boat(including Rumi Aikawa climbing down the stairs and giving you a Super Grenade Launcher), one in the boat, and five more hanging above when sailing the boat.
The other 10 are during the boss fight. Get to second stage of the boss fight(where the big gun comes out), and just hang to the right. If you hug the right edge of the screen, you won't be hit by anything. POWs will pop out from here, and will be rescued once you touch them(they're not bound, so no need to cut any ropes). At countdown timer mark 30 you should have gotten all 10.
This achievement is also cumulitive, so if you save 4 during one mission, then 6 during another, you'll also unlock the achievement. Also note that it doesn't matter if you die: the counter increases every time they're released, not when you finish the mission.
Run this route five times, and you'll unlock this achievement.