Is there any captive breeding of the diamondback water snake? I would absolutely love to keep one in a paludarium, but I could not morally justify keeping any species of wild caught reptile if you are not planning to breed it.
It may be morally better to not breed! This species produces dozens of young and they can be difficult to find homes for the babies. And, contrary to what is often asserted, breeding animals in captivity does nothing for wild populations. So please feel free to keep one snake and not breed it.
Second, Nerodia tend to get full of skin blisters when kept in damp or wet conditions. In the wild they stay high and very dry basking in the warm sun. Paludariums are very difficult environments to create in general, and harder still to create for a water snake. This species would do much better in a large, very dry enclosure with a water bowl big enough to easily soak in.
They are a cool snake that should be more common in captivity. Unfortunately they are most often wild caught and either have heavy parasite loads or serious skin diseases. You definitely want captive born babies with this species