I realize this is an old post but probably still very relavent.
Every spring when I open the pool I get 2 mallards. Usually a male and female.
This year 2 males.

In the past I have used a blow up swan and it worked well. Apparently ducks don't like swans.
THis year however they seemed to enjoy the swans company. I have pictures of them swimming with the swan, If I can get my computer working I'll post them , it's funny. On the wifes laptop now.