Those are baby black bellied whistling ducks, if they were in a swimming pool they probably hatched nearby and jumped into it if it's tall. Are there any trees with cavities near it? Or wood duck boxes? Whistling ducks are cavity nesters.
They're tricky to get to start eating so I'd either get them to a rescue or order some Mazuri exotic gamebird starter and get them started on that. They can also be given live mealworms to get interest in eating.
Keep them away from your other birds, because your birds can actually make them sick. Wild ducklings are usually pretty healthy and wouldn't be carrying anything that your birds wouldn't have been exposed to by other passing-by wild birds already.
Also, put them in a plastic tote with a heat lamp that they can get away from, a typical chick brooder. Do not use cardboard, they can climb it with their sharp nails. Use paper towels or regular towels as bedding, and use a water dish they can't bathe in. You want them to stay dry this young. Then sprinkle starter on the towel in a corner or toss in a few live mealworms.
