tomaO2
Planter
What about trying one of the small watering holes? Each corner has one. You picked that because it's the largest body of water but maybe they'd do it elsewhere. Could also be that the duck wants to eat first? Maybe there is a thing that if there is no grass to eat the duck doesn't want to swim.

, So I created a beach farm just to test it out, I only have 4 ducks and just got the coop for 7 days in game and I already see the ducks swimming, probably just the four corners farm layout causing the issue 