I let my ducklings swim fairly early. Here are some things I did.
Water only up to their bellies until they started getting real feathers which they oiled.
Water about 90F, warm not hot
Clean dry warm-enough brooder ready for them immediately after their swim (this was in February, by the way)
Help those that don't start preening right after their swim by wiping them with a clean dry washcloth.
Duck vitamins in their water afterward, since the swims were a bit of extra excitement and energy expenditure
As soon as they start looking tired (could be three to ten minutes), out of the water and rest. Tired means they start to slow down, one or two just stand there.
Watch over their every move every second they were in the water.
And we all had a blast!!!!!