Glory
Considering drowning is one of the leading causes of death in children, you're doing a public service. You're saving lives by teaching kids how to swim (source).
Kids will always remember you as a hero.
The satisfied look on the face of someone who never thought they'd be able to swim when they actually get it is pretty rewarding, even though you won't win any medals for being a teacher.
Your students, however, might one day win medals for swimming...and maybe they'll even thank you in public one day. In all likelihood, though, you'll just have to make do with the thank you a grateful parent gives after they see their kid swim across the pool for the first time.