Years ago I heard the term, "harness the inevitable." Ducks splash.
So, I have found that having a large straight-sided stock pot for water greatly - I mean, greatly - reduces splash. But does not eliminate it.
Another way to greatly reduce splash and slop is to put that stock pot inside something the ducks can walk into, and then put absorbent material under and around the pot inside the watering station. Mine is the lower half of a large plastic dog crate. The ducks step into the crate, drink and splash, and by the time they walk out most of the drips are caught in the sawdust pellets, which get changed as needed - usually after three or four days.
I have nine runners, and use a five quart stock pot for their overnight water.
The enriched sawdust then goes onto the compost pile or around the edges of the garden beds.