When they stopped eating their feed did they have the diarrhea at that point? or did that start shortly after they were only eating the eggs oats and cucumbers? How much of the oats, eggs and cucumbers were being offered before with the feed?
Edit: if you were feeding a lot of the eggs oats and cucumbers before with their feed that most likely is your problem. Ducklings are like kids and those extras are like dessert to them. When given the option they will stop eating the feed and start to only eat the extras such as the oats, eggs and cucumbers. This leads to nutritional deficiencies, lethargy, digestion problems, leg problems, overall feeling unwell and even death. Not to mention those cucumbers are very high in water which will make their stools very watery.