No duck really needs a pond, per se, but all should have access to water deep enough so that they can get in and get wet. Not only do they clean their eyes & nostrils by dunking their heads in water, they also keep their feathers in good condition by bathing and preening. Ducks who don't have adequate water for bathing are going to be more prone to external parasites which are nearly unheard of on a duck in good condition and they won't be as waterproof and they won't be able to maintain warmth as well. It doesn't have to be a big deal, a small rubber livestock feed tub would be sufficient. I use any number of things, but for a small flock what would work exceptionally well is a contractor's pan that can be found at Lowe's or Home Depot. They're in the cement section and are black, rectangular and about 4" deep, I think. Presto! Instant bird bath! I paid $4.99 a few years ago - do not know what they cost now although I do know they do have larger ones also available now which they didn't when I was buying mine.