Somewhere way back are posts on a container load of these spilling in mid ocean; they then were used to measure ocean currents.  Another thread covers an annual toy duck race.  Another covers a large inflatable one.
These ducks were used to illustrate an item on a baby penguin.
I guess that they are plastic, not rubber.
161212M Melbourne 'Herald Sun' - fairy penguin with rubber ducks.

Muppets 'Rubber ducky': www.youtube.com/watch?v=cDy4PZPMDwU.

Roderick.

180329Th Melbourne Herald Sun - rubber duck.

Roderick.