You should understand the osmoregulation process first and then you will understand what is the danger in case of drinking salt water.
You should know that osmosis lets water to enter in cells as a part of osmoregulation process. If the isotonic state is not reached, the osmoregulation process starts acting, hence, if you drink salt water and you manage to keep it inside (usually, the body rejects salt water by vomiting), the salinity oustside cells increases and the osmoregulation process makes the water inside cell to move outside, thus you will become dehydrated.
You know that many functions of our body rely on water help, hence, if the amount of water in our body decreases, these functions will be affected.
If you think that drinking pure water is then a solution, you are wrong again, since the isotonic state of our cells is maintained at about 288 milliosmoles per kilogram and if salinity outside cells decreases, then osmoregulation process moves the water outside cells into the cells, provoking an increase of the normal volume of cells and, finally, the explosion of cells, causing the death.
Drinking salt water is never a good idea-it can lead to sickness, dehydration, brain damage and death. Although some claim there are cleansing benefits to drinking salt water, the negative side effects and healthrisks can be far too great.