Duck's

Like water off a duck's back.

That's a clear case of "water off a duck's back".

It's like water on a duck's back.

The handle is shaped like a duck's head.

That's water off a duck's back.

For some people rejection is like water off a duck's back.

The duck's beak is also the rabbit's ears.

Criticism and accusations are like water off a duck's back to him.