I do a similar thing with mine. I used a half gallon milk jug and cut a hole in one side that I use for the water. They usually stay out of it, but I let them swim in the sink almost every afternoon. I clean their water out everyday. I usually clean their brooder when they are swimming. Then it is blow dryer time. But they are still messy. I'm trying to work out some ideas to make them a bigger area so that I don't have to clean it everyday. Every couple days would be nice.
Ugh. Ducklings are so freaking cute! Soon they will be outside splashing around in anything resembling a puddle.