I have four young ducks (almost 5 months old) that I got the end of April. I lock them up at night, in a large plastic airline type dog crate, so that predators cannot get them. I do not give them food or water. They go in at dusk and come out in the morning, at first light. When I let them out, they are fine, but do go right to the water dish and get a drink. I don't think it will hurt them. It's approx. 10 1/2 hours for them, now that I just added it up. It gets dusk/almost dark around 8ish, and I let them out at 6:30 or so. It does seem like a long time. I know that if they had food, they have to have water, but if they do not have access to food, I think they are probably ok.
I will wait to hear more from more experienced members, along with you. My duckies seem very healthy and are growing, happy, and very active, so I think they are probably ok.
Kathy