While sailing in the North Atlantic, large ship convoys will appear (designated by a raised fist icon) usually guarded by two frigates that will become hostile if engaged. The ship with prisoners onboard will show a counter of 100 and that number will drop if that ship takes any damage. Sink the escorts and you will be able to board the POW ship. It's possible to get this achievement in only 3 convoys, since 3x100=300, but will most likely take 4 in the likely event that you will accidently hit the POW ship with a broadside or mortar shot. If you're having trouble finding the convoys, search for them in the medium-hard difficulty large open areas of ocean.