I don't have ducks right now but here is what I did..
The left end is hardware cloth over a 5 gallon cement mixing tub. Then i use a submersible aquarium pump to constantly fill the waterer. The tube fits right into the water exit screw the top partially on to hold it down.
Not only does it drasitcally cut down the water mess, it will keep them watered for longer.
It is not hooked up but in the second picture you can see the tube laying in the bottom. I will hook it up in the next day or two when done using the save a chic stuff.
I always put some kind of platform under the waterer, depending on the size of the waterer. I have used two piece broiler pans covered with a washcloth, large steel salad bowls, shallow buckets, and the bottom half of a large dog crate (that worked the best) with some sawdust pellets in it to absorb water. When the waterer needs to be refilled, the splash catcher can be emptied at the same time.
Remember the ducklings need to be able to wash their little heads to prevent eye and sinus infections.
I use a towel under the waterer the first couple of days, then I switch that to a painters tray (for paint rollers) to catch the mess.
I recently built a big bird version of the rabbit wire over a plastic tub for outside in the coop. It has a lidless storage tote under (the flatter under-the-bed kind), built the box frame to match it in size. About 2 1/2 x 1 1/2 ish. I didn't know how much water those ducks were throwing into the bedding until I started catching it in the plastic tub. Good heavens it was a lot! Now I dump it harmlessly in the yard. The ducks can step up onto it and sit by the water without being able to go swimming.
If I can convince my husband of the need for a duck brooder AND a chicken brooder, we'll build one with duck specs. Built in wire floor with a pan under it. They can then make as much of a mess as they want and they don't get to re-use it. I was thinking the whole bottom as rabbit wire, though half would have the towels on it as a soft spot to sleep.
Sadly the extra building supplies went to the rabbit hutch. So, paint tray it is until they graduate to the great outdoors in the coop.
But the rabbit wire over top of a container, amazing mess catcher. My bedding stays dry until it rains, I have a leak I need to see to from the windows. Unless you count wetness from poo, I grab that up daily. But between the ducks, chickens, and litter box trained rabbits, I go through a 5 gallon bucket (and a half) of dirty pine bedding. Drastic reduction.
OOooo, I bet I have enough scrap to build another one to fit the brooder and I can use a rolled up towel as a step onto it for the new little ones. Use my paint tray under it and build a little bird size!