Many people's answer is that the salt produced by sea water drying cannot be eaten directly without being purified again, either because it contains too many impurities and is easy to have diarrhea, or because it tastes bitter and affects dishes. Please, the premise of this question is "on a desert island" and it has drifted to a desert island. It's good to have salt to eat. Do you still care about impurities and bitterness?
Let's talk about impurities first. It is undeniable that the salt baked in seawater contains molecular compounds such as magnesium chloride, calcium, bicarbonate, sulfate, bromide and borate in addition to sodium chloride, which is really not conducive to long-term consumption, but this salt will not cause some strong symptoms.
To give a very simple example, back 20 years ago, almost every household in the north would pickle pickles in winter, and the main materials were radish, kohlrabi and ginger. The salt used at that time was all unpurified salt, which is what we call "crude salt". I haven't found any serious symptoms after eating for so many years, and it is even worse to eat emergency on the island.
Besides, it tastes bitter, because it says that unpurified salt does have a high impurity content, so it does have a bitter taste, which is known to those who go swimming in the sea and drink seawater.
However, the salty taste of sea salt is greater than the bitter taste. Imagine eating wild fruit after living on a desert island for a week, or eating barbecue directly without any seasoning, just like eating "Deye" and "Beiye", a bird fades out of your mouth. At this time, who will care about the bitterness in the salty taste? What's more, the taste of wild fruit and barbecue itself may be more difficult to swallow than salty and bitter. If you don't believe me, you can roast a piece of pork and chicken without seasoning.
Besides, living on a desert island only eats this kind of salt in an emergency, and you can eat whatever you want when you go home after the rescue, so don't worry too much.