Hiya, This has been discussed quite a bit lately, so it's worth doing a search.
Yes, they're messier than hens, but if they have a decent area, don't keep too many, and clean them out often, they're fine. Mind you, ours have 25m2 each at the moment, which may be a bit excessive!
As a minimum for health, they need to be able to dip their heads in water, but yes they love to swim - so would you if you were a duck!
Oh, and re keeping them together, probably ok on the same ground depending on how much you have, but keep different houses.