Consistent front-back swimsuit color makes kids easier to track in the pool
Richard's hack is one that would never become apparent unless you're in the situation:
I made the mistake last year of buying my son a set of swim trunks that are red on the front, but black on the back. This makes him MUCH harder to find in a crowd, especially since every other piece of clothing he owns is color consistent front-to-back.
A brightly-colored swim shirt also helps make a kid spottable in a crowded pool, which is especially useful when you're keeping track of more than one kid.