A rubber duck was a duck-shaped toy usually played with in the bathtub. They were sometimes made of plastic.
The Doctor had a rubber duck which he sometimes kept in the TARDIS swimming pool (PROSE: Evolution, Reckless Engineering) and sometimes in the bathroom. (PROSE: Lungbarrow, Seeing I) Shortly before arriving on Olleril, the Seventh Doctor played with his rubber duck in the TARDIS jacuzzi. (PROSE: Prelude Tragedy Day)
The Sixth Doctor, while visiting Devon in 1929, had a rubber-duck shaped key in his possession. (PROSE: Timeshare)
The Doctor's house in Kent had a rubber duck in the bathroom. (PROSE: Cat's Cradle: Warhead)
Besma Grieve owned a rubber duck, (PROSE: The Year of Intelligent Tigers) as did Vasar. (AUDIO: The Draconian Rage)
The Eleventh Doctor pulled out a rubber duck while searching for his missing sonic screwdriver. (COMIC: Sonic Sleuth)
When the Nestene Consciousness took control of all the plastic that had collected in the North Atlantic garbage patch, the rubber ducks were animated and made to fly. They attacked and sunk a lifeboat with two sailors trying to get away from a container ship under attack. Later, after torching the ship to subdue the alien threat, Rhys Williams and Kaitlin Russell were terrified they might get attacked by rubber ducks themselves as they awaited rescue. (AUDIO: Sargasso)