I am a teacher and used to have to leave mine overnight with similar results. I did what the others have suggested, took a gallon milk jug and cut out holes only big enough for their heads, not their bodies, about half way up. I found there was still water in the jug in the morning and I usually have 4-8 ducklings at a time. Move the food a little ways away as well. Ducks love to get a bite of food, then a drink of water, then food, etc.. But, they dribble that water everywhere in the process. If you put a little distance between the two it helps some.
If you want a good, simple book about duck care I really liked, Duck Eggs Daily: Raising Happy, Healthy Ducks . . .Naturally, by Lisa Steele.