The ducks are usually pretty good at finding a suitable (in their opinion!) nesting site but they do need protection from foxes (sitting ducks) The ones here mostly nest in the bedding in the animal sheds, though sometimes choose my plant containers The island would be a good solution. The drake will keep watch over his duck,(() so needs a platfom with room for him to sit. The eggs are laid over a couple of weeks. Some get carried away and lay too many, more than they can successfully incubate. 6 to 9 ducklings is average. Incubation takes about 4 weeks.
However the ducklings, once hatched, do not spend all their time on water and on land they are very vulnerable to crows and magpies and quickly get picked off. I generally shut them up in an enclosed pen, as it is the only reliable way to ensure their survival, the percentage from free ranging ducks is very low. This probably won't be possible for you, with a wild duck, but feeding her may make her tamer. You will have to resort to crossed fngers and toes and if you are lucky may end up with a team of slug catchers!