Consider a paint roller tray as a pool.  The graduated depth makes it easier for the ducklings to get out of the water.  Also consider putting a piece of hardware cloth over a shallow baking pan, bending it over the edges and putting the waterer on top in the center so splashing gets collected in pan.  You can build up bedding around the pan to help the ducklings get up to the water.  If the waterer is too heavy for the hardware cloth, put a dish or other object underneath, inside of the pan, to support the waterer.  Put bedding on top of hardware cloth to protect ducking feet.