You will find them much much messier.
It will be best to keep them separate from the hens unless you choose to just have 2 or 3 Khaki Campbells to keep in with them. They were a breed specifically bred to be kept with hens and also for their good laying qualities - Kotlang (sp) do the best commercial line I think - I only ever kept ducks for meat (don't like the taste of duck eggs)
Your ducks wont need swimming water unless they are a really heavy breed, and then only for breeding, but they really do enjoy splashing but an old paddling pool or childs sand pit will do the job and be easy to empty and clean out, which will need to be done frequently.
Ducks dont need protection from the weather so dont need shuting in at night but they are at big risk from foxes etc, and baby ducks will be taken by rats and crows so need special protection
Could say lots more, but need to get food cooked. Ask if there is something else you need to know and I am sure others will be along soon as well