There's a very high probability then that this is exactly what happened. :( Snapping turtles can be very dangerous. They eat ducklings, and can drag a full-grown duck under, too. Some people eat snappers, so maybe you can try to catch it and turn it into soup?
The original wound may have...
Ouch, poor baby. Unfortunately, at this point I don't think it can be joined back together without surgery. The duck should be fine once it's fully healed.
I would keep applying some ointment on it and keeping it clean, perhaps some confined time for about a week. Don't get it wet if...