How often do you feed them and how big are they?
If they don't eat it within 15 minutes or so - just fish it up again and try later/next day. Keep some aquatic plants in the tank, then they have something to eat if they're hungry.
I've never had any problem getting my turtles to eat almost right away. Most food barely hits the water before it's in their mouth or they're chasing after it

When I made my own food (mixed fish and sallad in gelatin) I used feed it in the sink because it got so messy.