The girl cannot see the actual underside, but she can see the water, which is where the reflection appears. The required length of mirror is exactly half his height. The distance from the mirror does not matter, because whatever distance he is in front of the mirror, his image appears exactly that same distance behind the mirror. Least Time Lifeguard must rescue drowning person. Each path takes the same amount of time.
Path is bent toward the normal.
Path remains unbent. A coin is under water.