It's so fun when they swim under the water like that!  I remember my little Mel, mallard duckling I found under the brooders at the feed store and they told me to take him home...  anyway, Mel used to do that when he was about a week old.  I let him swim in a bucket...   round and round and swooop! under the water for a few laps.  He loved it!  Didn't want to come out, he would dive every time I reached for him.  Not afraid though, he was imprinted on me.  Lasted until he was mature and we got him a girlfriend.  I could still pick him up any time.