White and runny poop is pretty normal for ducks and ducklings. Ducklings really don't need to go swimming until they feather out. You need to make sure that if they get wet you dry them off very well so they don't get chilled, as this can kill them. You might try putting some vitamin/electrolyte mix in its water. Otherwise, I'm not sure what to tell you. You might look on some of the duck posts to see what others advise for sick ducklings.