This is NOT a good method. I'm a dental assistant. Using anything this abrasive can ruin the enamel on your teeth which can lead to some not so nice things like sensitivity or even cavities.
Ughh!!! Don't listen to dentists they tell you no because they don't want you to do that just so they can make more money!!!
Using sea salt is an excellent way to whiten your teeth. Use it twice a week instead of using toothpaste. Rinse out your mouth well after just so your don't swallow it. As with anything, check with your dentist first in case there is a reason why you shouldn't try this method.