Cutting the grape in half helps your pet to chew on them better. That way, it makes it easier for him to swallow and digest the grape. You should only feed your omnivorous turtle a little bit of fruit and vegetables, such as water lettuce, now and then. These foods should make up only 10% of your pet's diet. Feeding grapes to turtles now and then will not cause any harm. However, just like us humans eating chocolate, it's best not to overdo it.