In 30 years of keeping ducks I've never had a case of fly strike! I had a hen with fly strike once but that was because she had been cut by the roosters spur under her wing and I didn't notice straight away... I washed it with salt water and gave her and antibiotic and she was totally right as rain within 24 hrs! As long as your ducks are able to wash or swim and are not wounded they should be fine, don't worry!