We humans can eat saltwater products. We are physiologically able to process salt, via our loop of henle in our kidneys.
The Henle's loop in a reptile is quite short. If a freshwater turtle consumes salt, it can damage the turtle's kidneys. Too much salt will lead to renal failure.
Some reptiles have developed alternative methods to deal with salt. In the case of the Kakadu and PNG form of Carettochelys insculpta, they have salt glands. The Daly River form of Carettochelys insculpta and all other freshwater Australian turtles do not have salt glands.
So my advice regarding feeding saltwater animals to freshwater turtles is simple: don't do it.
I know that we can eat saltwater animals with no ill effects, but we are not reptiles. Koala's do quite eating nothing more than certain species of Eucalyptus leaves, but I don't. I can eat onion, garlic and chocolate, but these foods can be bad for a dog.
It is important to not become confused with food being consumed equalling nutritionally good food. Just because a turtle eats the food you give to it does not make that food good for the turtle.
Regards,
Michael.