Do you have a small ark or similar you can put them in, then get the broody back with them? She will provide just the right conditions to rear them. She may simply have decided to dump them on you temporarily while she went off for a swim and a feed. Then when she came back, they'd gone.
With mother raised ducklings, they get moving pretty quickly, and can jump (float) up onto things within a couple of days.
I use a seed tray (with no holes in the bottom) for water and paddling, with a stone on the outside and another inside so they can get in. Initially there's hardly any water, but as they get bigger they get a bit more. It does have to be changed 2 or 3 times a day, but so - you'll be looking at them that often anyway. As well as chick crumbs, they will soon need some greenery such as chick weed. They will also soon need a separate water bowl - we use a heavy glazed pottery dog bowl.
They do c**p a lot, but you'll be used to that from the adults.
We have always raised our chicks and ducklings in the polytunnel initially, but I do have to sacrifice some of my plants to them.