Seawater is almost deadly to the human body. It contains such an amount of salt that the kidneys would not be able to excrete it without a large amount of drinking ordinary water. Which the castaways don't have at their disposal. In this case, it is much better to drink your own urine, ie when it comes to saving lives. It is also salty, but it does not have such a drastic effect on the human body. And there's still a lot of sodium chloride in the seawater, or something, I don't know now. It is simply not allowed to drink seawater to quench your thirst.