if they are feathered up, then they are fine, ducks can handle the coldest weather you can throw at them.
As for the above post, they are correct about swimming at birth, the deal is, they have to be hatched my the mother to be waterproof at birth, her oily feathers gets on the ducklings down while they are under her, thus waterproofing them. I have raised over 110 types of migratory /ornamental waterfowl for decades, so kinda know a thing or two about them. The oils come from the normal preening and cleaning of their feathers, the more exposure to water they get, the more they'll do it, so short trips once a day are great, doesn't take long, but if their feathers were soaked, that's what the problem was, they didn't really drown, just got waterlogged, and chilled....When they are cleaning themselves, watch them, you'll notice they keep going to a gland found just above the tail bone( center lower back) that's where they get their oils from. They will take it and smear it all over themselves..
But you do need to expose them to short trips to water just to speed up the process, if they are already feathered, they usually are okay, but some just need a little more time. Also, if they are in that "ugly duckling"stage, between down and feathers, they are more susceptible to this, as the oily down is falling out, and new , unoiled feathers are coming in. This too, allows them to be more prone to holding water...
But don't get discouraged, they are relatively easily kept, and once you know what all to watch for, you'll be just fine with them, once fully feathered, you shouldn't have any more problems..
Good luck
Aubrey