Duck lost entire chest by turtle attack - help!

I don't want to sound harsh, but I don't want to offer false hope.  Ducks can make miraculous recoveries, but you need to be aware of some things.

One critical item to take care of is getting the duck into an area with no flies.  They will lay eggs in the open wound and add horror to horror and pain to pain.

If bone is exposed, that is a very rough thing.  My vet told me that exposed bone can get infected and be impossible to treat.

If bones are not exposed, and you quickly get the duck away from fly risk (think indoors, in a cage with screening around it, here is what Storey's Guide (as I recall) recommends:

get the duck to a clean quiet place away from flies.

clip the feathers from around the wound

put something like sevin powder very very carefully on the skin and feathers around the wound, not in the wound

The Ultimate Pet Duck Guidebook says:

If your duck has any lacerations or bite marks, you are going to want to use a saline solution to do a quick flushing of the wounds. Then use gauze pads to apply direct pressure to stop the bleeding.  Once bleeding is controlled, you can lightly spray the injury with a non-sting wound wash.

If you have Baytril on hand, give our duck one 22.7  mg antibiotic pill to help prevent infection.

Get the duck to a vet.

.......

Addition:

So I would flush the wounded area, clip feathers back at least half an inch away from the edge of the wound, and examine.  If I knew a vet or anyone with the ability to help, I would call on them immediately.  I would flush the wound with saline solution and put sevin around the outer edges, very carefully.  I would put a light coating of triple antibiotic ointment (not pain relief type, it is toxic to ducks) on one side of a gauze pad and hold that in place by wrapping the duck with vet wrap.

I would put the duck in my sick bay in my house, and call a vet.

Amiga is always so awesome in her advice! I am so sorry about your duck. <3

One thing you can do for the future is try to find and remove the turtle if you can so you no longer have any more ducks being seriously injured because of them. There's a small pond by my house and I had raised 2 ducklings to mature adults and released them at the pond, we had no idea the pond had snapping turtles in it and one of the ducks had half of his beak torn off by a large snapping turtle. It may sound like a crusade but after spending days trying to remove every single snapping turtle out of the pond and relocating them miles away we no longer have problems with our ducks being attacked by snapping turtles. It's a sad freak accident and I hope your duck makes a full recovery.

You could also try cayenne pepper powder on the wound. It sounds like it would be painful, but it really isn't. It gives the body B and C vitamins, as well as stopping bleeding. Plantain would help it heal, you could apply a poultice and feed some to the duck as well.

How are things going?

Just to give you some hope, my muscovy drake got ripped open across his chest and had two small puncture marks that I could see.  He had stopped bleeding by the time I found him.  Someone recommended flushing the wound with diluted hydrogen peroxide and then apply neosporin without pain relief.  I had him in my spare bath tub for several weeks and kept the wound clean.  When he was healed fairly well I applied blu-kote spray and put him back out in the yard with the other ducks.  It has been a couple of months now, I think, and he is all healed up with new feathers growing back in.  I hope it turns out well for you and your duck. 

Glad to hear she's doing well!  Don't let anyone tell you that you're crazy to spend that much money on a duck.  I don't think it's crazy at all.  One thing I've noticed is that for some people, ducks are just livestock or decorations.  For other people, ducks are pets.  Neither perspective is right or wrong, but if you consider a duck a pet and a family member, then of course you'll spend money to save her!  I'm happy to hear a full recovery is expected.