The real question is under which circumstances is it helpful to drink some seawater in addition to fresh water. Suppose your solar stills can generate 1.5 liters fresh water a day, is adding half a liter of sea water a good idea in that case? That would make for 0.75% salt content. The blood and presumably sweat is 0.9%, the water lost through breathing and evaporation directly through the skin 0% and urine apparently max 1.5%. However, the water lost also includes water generated from burning fat, sugar and proteins, so you should expel more water than you drink.
Your presumption is wrong, the salt concentration in sweat is much less than that of the blood. A much smaller amount of seawater would be adequate to replenish lost electrolytes (if that were a sensible way to replace electrolytes, I don’t know about that).
There are also places in the ocean, near the mouths of major rivers, where the water is much more dilute and in some cases drinkable. I’ve heard that for the Amazon, this drinkable-water zone extends beyond the point where you can even see land.