I have actually witnessed this topic with my own ducklings. My first brood I took them from momma right away and raised them myself. I noticed immediately that when they get themselves wet their down would water log easily. I can see how if 1.left unsupervised And or 2.unable to get out of water and or 3.getting chilled and not being able to warm themselves could have tragic consequences.
My second brood stayed with mom. Due to (what I have since then learned) the oil transfer from her feathers, I didn't have to worry so much about them getting wet as long as they could easily get in and out of the water. I could visibly see water running off of them unlike the ones I had raised without momma duck.
That being said would personally still be cautiouss about ducklings having access to deep water unsupervised even with momma duck around. If one would happen to get tired and unable to escape the water the mother would be unlikely to save it if she has her other ducklings to tend to.
It sounds like this Op is interested in bringing home ducklings that will not have a mother or adult to look after it. I would recommend supervised swimming until feathered out to ensure they are able to escape when they become tired and get dry and warm wether that means you dry them off or they have some heat source to warm up and preen themselves under.
