You need to do what seems right to you, but I'll tell you - there is a creek just down the hill from us (not on our property, but close).
My ducks could care less. They have a very nice, spacious pen, go on walks with me every day, concrete mixing pans with fresh water to swim and splash in...
I would be more concerned with predators and the ducks just swimming away.
It partly depends on your management - and the ducks' personalities. Some folks can establish that the ducks' home is in the duck yard, and though the ducks visit a water body, they come in in the late afternoon for...