A few towns over from me has a an island house for them, the only way to get to it is to swim, so the ducks seem pretty safe.
They need to be put inside something in order to be really safe, I dont suppose it matters what it is, just as long as nothing can get inside it.
My neighbor bought about 20 ducks to put on his pond, he left them no shelter(He diodnt want to go outside and close them up at night) and he lost all 20 pretty fast. Since they cant fly away they will need to be shut up at night.