Some of Fortnite's 14 Days of Summer challenges have been interconnected; for instance, one asks players to destroy grills, but Epic Games requires that this be done with a harvesting tool you only get by launching fireworks as part of another challenge. Another example of this involves a tiny rubber ducky, which you're told to search. The only clue given is that it's hidden in the Summertime Splashdown loading screen. [Update: The clock is ticking and 14 Days of Summer's challenges will soon expire. You can still do all 14, with each one netting you a reward; complete them all, and you also get your hands on the exclusive Smoothie back bling. You actually don't need to specifically do this one anymore--rather than change how it works, Epic auto-completed it for players who log in during the qualifying period.]
If you're at a loss for where on the map to go, you can check out the video guide above for a walkthrough on its exact location. You'll find it in a tub south of Paradise Palms, but be prepared for a real rush--at the time of this writing, there's only one duck that spawns per match, and with other players trying to search it, this can be very difficult.
A fix is on the way, though we don't yet know when. On July 7, Epic tweeted, "This challenge is harder to complete than we intended. We're working towards a fix in a couple days." There has been no further update on that yet, although content update 3 for patch 9.30 is out now.