By 2-3 months old they should be okay, I agree with ruthhope, they mustn't have been able to get out of the kiddie pool. And probaby had wet feather? If they become watedlogged it'd be harder to get out.
With free ranging, there will always be a predator somewhere, whether it's a hawk or cat or...
It depends on the age of the ducklings, the ones in this thread are old enough to preen and waterproof- so are safe to swim. Young ducklings should not, as they can get waterlogged and chilled, inable to get out. Ruthhope isn't wrong, she just isn't talking about the same age group as you...