in case one ends up in flowing water
The Rubber Duckies were part of the Ghostbusters survival gear inside their Ghost Packs. They were purchased by Eddie Spenser Jr., who was supposed to buy life jackets for all three ghostusters, but found Rubber Duckies on sale instead.
Jake Kong Jr. and Jessica Wray did not know about the Rubber Ducks in Jake's backpack until they needed to use it after falling into a moat under a haunted castle in Limbo Land, where they were lured to by entering a magic painting.
The rubber Duck first appeared in the episode Really Roughing It.