If a shipwrecked crew drinks sea water, they will probably die. why?

They will die because the salt will be of an excessive manner to their bodies and therefore will dehydrate their cells thus, killing them from dehydration.

There are two problems.

First is the osmotic problem of the high salt content, but second, sea water contains high amounts of sodium and other chemicals that can cause diarrhea, leading to even more water loss from the system.