The bottom line is that you can maintain and develop some swim fitness with cords.

The more you try to imitate the aerobic and muscular demands of swimming the better it will work.

There are a whole list of problems with this approach however.

List of problems:
1) It feels like a strength activity. Therefore, you treat it as one and don't bo aerobic activity at all.
Solution- don't forget to do regular aerobic sets.

2) You won't get the feel for swimming.
Solution- try to use arms as you would while swimming.
Pretend you are swimming in your minds eye.
Make sure you get as much real swimming in as is practical.

3) Most arm power is deployed under core while doing real swimming.
Cords will overwork triceps.
Solution- adjust cords to do occasional strength exercises directly under core.

4) Swimming also requires specific core strength and leg/kick strength.

I don't have a good answer to this?