Ducks Unlimited Web page has this.

Link
http://www.ducks.org/hunting/waterfowlgallery.aspx

And going to a local place which will have lots of ducks such as a park and just sitting and listening will help you too. But I beleive many over call and call too loud.

On any Mallard type duck call you can learn to call Mallards of course and use it to simulate a Diving duck call easily. it is all done by tone.

The other call I use is a Pintail/Wood duck/Teal whistle.....

JW