Actually, overconsumtion of water can be fatal in young ducklings is something I have seen mentioned before on a how to raise duckling page.
If you are really worried about it, pull their water for brief periods of time as I have seen it mentioned that every once in a while you may have ducklings filling themselves up with water instead of consuming their food. You may even want to entice them to eat by giving them mashed hardboiled egg to see if you can increase their appetite. Adding vitamins to their water sounds valid. I would definetely check the temp in their brooder, as duckling don't need as high a temp, they feather out quickly and out grow having a light far sooner than chickens do.
For me, my waterfowl always seem to be eating and drinking. They love to spit food into the water dish, if yours aren't constantly doing both and growing at an unbelieveable rate, you should definetely monitor them.