Well, I've got sheep, chickens, dog, cats and ponds (one of which is really a mini lake and is about 10 feet deep). Sheep prefer to drink from accessible sites....so, if the pond has a gently sloping edge, they'll drink from it. Sheep seem to prefer clean to dirty water (unlike the dog, cat and chickens who seem to be the opposite). Sheep which do not have overly long or thick coats which can get waterlogged are able to swim for short distances, but they won't be swimming as a hobby (only if they fall in).
Sheep and chicken get on fine....but the chickens WILL steal any grain/pellets/ewe or lamb nuts. Sheep are also very happy to eat chicken corn or layers pellets given the chance. I'm sure ducks would be fine,too.
As norseofcourse says, boggy areas predispose to gastropods (slugs and snails) and a small boggy-soil-loving snail is the secondary host for liver fluke.If liver fluke is prevalent in your area, then flukicide will be needed regularly in the warmer weather, especially after rainy periods.We have a very we climate and a chronic fluke problem, so we have to use flukicide 2-3 times over the summer period, depending on the temperature and rainfall.
Lambs can be a bit of a problem in spring with the chickens as they chase them in play.....at least my chickens get slimmed down a bit then.