They are messy,
if you want your chickens to have clean water keep them separate. Ducks wash their food before swallowing it and dibble around in the earth too, so their water is never clean!
You can do little about that.
They need ground floor accommodation, will use a nesting box about 25% of the time and the great outdoors the rest of the time so you need to be aware of egg predators - I have a magpie problem, some people have rats. They both take the eggs away so you never really know if they are being stolen or the ducks aren't laying.
If you can persuade your ducks to go inside at night and leave them in till about 10 am you are likely to get all the eggs as they usually lay fairly early. They need to be able to put their heads under water regularly to re-oil their feathers. I have a small stream running through the garden, but a duck pond is just as fine but it gets very dirty as they poo in it while swimming about, so you need to be able to empty it (and the disgustingly smelly sludge) They are great fun though. As to where to get them - ad trader type papers, probably googling 'Indian runner ducks Cambridge' might get a result, or ask a local farmer if he knows of any breeders, or the local Livestock market. Don't do any research - just get some! You will enjoy having them.