not to mention a solo duck wont do much for a pond. Ducks are flock creatures and need the company of other ducks. As Kevin mentioned some just dont enjoy water as much as others. To continue to traumatize the bird by throwing it into the water isnt a good idea. We have a drake who hates water, he will bath, but doesnt like deep water.
Often with natural ponds duck sense danger and refuse for good reason to not enter the water.
I just remembered my duck suddenly became afraid of the water and would not go into the lake at all. They would waddle around the banks and keep stretching their necks over the water. Even sneaking down to have a drink, then running back up the bank.
Then my ducks started disappearing at the rate of one a week. One night I found out why they were disappearing and why the ducks were afraid of the water..........it was a huge python! It was in the process of swallowing a duck - out of reach in the water.
After that I had to re home most of my ducks and just kept a pair as pets in my garden - away from the lake.
that doesnt mean you shouldnt provide proper housing, food, another duck to be with. It also doesnt mean throw the lone duck into the water. Not all ducks will "do" what the human want them to, this needs to be understood from the get go. Housing, feed, and all proper care should also be understood long before adding any animal to your care.