This achievement is easy to get in the starting area, but first things first.
You need to unlock the Shallow Water Breathing skill in the Shinobi tree for Naoe. It's on the bottom right.
Next, after you have started combat/are detected, you need to find water shallow enough such that you can essentially go prone in it. After a few seconds, she will pull out her reed and start breathing through it.
The place I got the achievement in is in Amagasaki area of the Izumi Settsu starting region. Look for a location called The Lumber Aisle (red diamond), which is near the water, southwest of Amagasaki Castle, and directly south of the kakurega there.
Simply aggro at least one enemy near the shore, and then go down into the shallow water where they can't find you and wait for her to stick up the reed.