I must agree with Joyfull here. I've kept hens for many years in a free range environment but branched out into having 6 ducks this May. I must say I just love them and wish I'd kept ducks sooner. I kept them penned off at first from the hens for quite some time for them to gain maturity and confidence. Ducks are much less pushy when it comes to food in comparison to hens. Now they are all grown up and super confident they free range with the hens but I always ensure they get their fair share of food e.g. afternoon corn as they remain less forward than the hens.
As for water I let them have a child's rigid plastic paddling pool. It is emptied and filled every day once as they just adore clean water and require it for preening and cleaning. The paddling pool is a muddy mess by the end of the day. I also ensure the pool is moved each day so the grass underneath does not become over waterlogged and has time to rest after the ducks have spent a day on it. You will need to seriously consider how much water you can give your ducks access to and also how you are going to dispose of the end of the day water. As mentioned the ducks and hens could very easily end up living in an environment that is muddy (mixed with plenty of poo) and the ground will become very quickly hen and duck sick.
My ducks als o have their own duck house and are extremely indignant if a hen goes to bed in there. Ducks are party animals and will stay out late long after hens have gone to bed and need to be put to bed at dusk.
I don't think they poo more than hens but the poo is
very wet and does not degrade as fast as hens poo on the ground. In their house it can get very smelly and wet very fast.
Good luck in your choices and I'm sure you will decide with the ducks and hens welfare as paramount.