Physical Danger
Swimmer's ear is a real pain.
You could also be looking at repetitive shoulder injury or a torn rotator cuff from overuse—however these things are pretty rare for a teacher and more common for an athlete (source).
Being the teacher who already knows how to swim, you're really not in a lot of physical danger...unless maybe you get a foot cramp in the middle of a lesson. But even then, it's not going to be tragic.
You do have to keep a close eye on your students, however, who could be in real danger of drowning if you're not careful.