If you are craving salt, you are likely dehydrated. Salt helps the body retain Water so your body craves it when it is low on water.
The first 3 days of your pre-op diet are the absolute worst, it will get easier from here. You've made it past those, congrats. My husband got a death threat from me on my third day, so I understand it is no joke. The pre-op diet is also the hardest by far. The only other time is after a fill or an unfill you will have a few days of liquids also. That is nothing compared to weeks on liquids only though! Can you call your doctor's office and see if you are allowed any eggs or something else that would be filling but wouldn't de-rail you? Good luck, the pre-op diet is quite the mental fight.