A good supply of fresh water is very important. Your ducks will drink more at this time of year so make sure there is always some about. Their swimming water will also need changing regularly to prevent algae from growing.
Let your ducks roam in the garden and they will pick off pests like slugs and flies. You may want to protect young shoots though as ducks can be quite partial to tender young plants. Your ducks may go broody around now; to prevent this make sure you collect the eggs regularly.