Coaches, you need to be careful when critiquing a swimmer's stroke from above the water. Here's a couple of examples why:
James has a low body position which we should work on improving:
However from above the water it doesn't look nearly so bad:
Michelle's dropping her wrist whilst breathing here:
Yet from above the water it's hard to detect:
Just like that caveman found when he first tried spearing a fish in a stream and missed, the boundary between water and air is very distorting. Objects and depths appear more shallow than they actually are. If a swimmer's legs appear 20cm below the surface of the water, you won't be far off by doubling your estimate. If you can detect a slightly dropped wrist or elbow, then it's probably significantly worse that it appears.
Fine tune your observation skills and your stroke correction abilities will take a big step forward.