Another thing to consider if you are in the northern hemisphere is you'll be going into winter. Not impossible by any means to raise healthy ducklings in winter, but it does complicate things a tad
Just a note that some domesticated ducks can lay enough eggs in a year to be competitive with some of the more productive chicken breeds. If they are in fact fertile and she's broody and you want to hatch them then go for it, but just be aware you may be getting a lot more eggs in the future