Drink urine or seawater?

Long time no hear, but now I am back with more random knowledge

I just bought a great book called "why do men have nipples"
I have decided to write up/albeit sometimes summarize when I get tired of writing/ some of the greatest points in the book.

If stranded on a desert island, should you drink seawater or your own urine?
Seawater is more than three times as concentrated as blood. Drinking saltwater forces your body to to deal with a more concentrated solution than its own fluids. As a result, your body must excrete it through the kidneys as urine. The kidneys can only make urine that is less salty than salt water, so if you drink seawater, you'll be peeing alot and losing an excess of water. This would cause your body to dehydrate, leaving an excess of sodium in your bloodstream. This again causes your cell to shrink and malfunction.
Muscles would become weak and ache, the heart would beat irregularly, you become confused and then you die...
Drinking urine is probably safer, but but the catch-22 is that you dont have any water to drink, you will become dehydrated and not produce any urine.