It didn’t. He had fresh water for most of his time at sea.
It wasn’t until he ran out, and became very thirsty, that he tried drinking sea water. He tried sipping it from clothes he dipped in the water to reduce the salt, it temporarily quenched his thirst but the actual benefit from *sipping it from his clothes* would have been that he didn’t drink very much sea water, and that would have reduced any ill effects.
He did that for four days, not long enough to see any of said ill effects. After that, it rained and he was able to collect more fresh water, which lasted until he was rescued.
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