Edit: Question answered. Thanks!
Don’t be too hard on me, I almost failed chemistry:'(
In: 2220
Seawater is much saltier than electrolyte solutions.
Seawater is about 3.5% salt. Your body tissues are about 0.9%. Your kidneys can concentrate your urine to more than 0.9%, but not to 3.5%, meaning that your urine is always removing less salt-per-water than seawater. If your kidneys work as hard as they can, they can manage about 2%. That means that, to clear the salt from 1 liter of seawater, you need ~1.75 L of water in your urine. +1 L of seawater, -1.75 L of urine equals -0.75 L of water overall.
A 20 oz bottle of Gatorade contains 270 mg of sodium. That’s an implied content of about 681 mg of salt. 20 oz of water is about 600 mL, or about 600 g of water, meaning that Gatorate’s salinity is about 0.1%. That’s less than your body tissues, and far less than seawater.
A saline solution pumped in by IV at the hospital is typically *isotonic* – equal to your body’s natural ~0.9% salinity – and would taste very salty if you tried to drink it.
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EDIT: a point that has come up in some of the comments here is that, because your body absorbs water by osmosis, you can’t actually absorb water from a 3.5% saline solution at all. This is true: seawater in your gut will simply pull water *out* of your body osmotically, and in fact laxatives usually work on this exact principle.
That said, adding 1 L of seawater to 10 L of fresh water would result in a mixture your body can absorb, but which would result in less hydration than just drinking 10 L of fresh water would, for the reasons described above.
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