Salt water is too salty and saline is just right.

When you are dehydrated you also lose salts/electrolytes which are not exactly the same as sea water, for example.

Normal saline is 0.9% sodium chloride.

That's 9 grams of NaCl in one liter of water. This is isotonic to your blood.

Sea water has ~35 grams per liter, almost 4 times as much.

https://ispyphysiology.com/2019/07/10/sea-water-vs-saline-why-not-all-salty-water-is-created-equal/

Amount of sodium is the biggest factor. Saline is purified water with a small amount of sodium chloride. Sea water is a high amount of sodium.

Its all about concentration. Saline is meant to match the body's natural level of electrolytes. It's the same reason sport drinks like Gatorade contain electrolytes. They can replenish fluid levels without disrupting the electrolytes balance.

Some important concepts

1. Salt sucks up water. Water draws out salts. This is known as osmosis.

2. Your body Is a bag of salty soup. We call these body salts "electrolytes". You need electrolytes to live, and the electrolytes need water to be useful..

3. Your body pees regularly to get rid of stuff it doesn't need. Your body will always need to pee, regardless of how many electrolytes that water holds

4. Sea water is MUCH saltier than Saline

In short, you need to keep a balancing act. You need a certain amount of electrolytes to function, and you need a certain amount of water to hold those electrolytes. Your body uses water for different things tho, so you need to keep that balanced.

When you drink sea water, the water is MUCH saltier than your body. Instead of your body absorbing the water, the salt sucks water out of you, and you pee it out. Your body has less water at the end of the day.

If you drink pure, unsalted water though, the opposite happens. You drink the water, your body is saltier than the water. The water sucks out salt from your body, and you pee it out. You loose electrolytes.

Now, you don't wanna lose water. Dehydration causes a tone of issues. You also don't wanna lose electrolytes. That can cause just as many issues

Saline is a happy balance. it's only a little less saltier than your body, which means your body can take in water, pee it out, and lose little to no electrolytes. This lets someone who's dehydrated gradually rehydrate, without a drastic change in anything else.

This gradual change is often really important, because your body isn't a machine. It slowly adapts to extreme changes like starvation and dehydration to help you stay alive in those states. Quickly changing their intake (like giving a severely dehydrated person a gallon of fresh water) can really mess their body up if they aren't gradually introduced to it.

Sea water is waaaaaaay salter than saline. We need enough salt(electrolytes) to be able to conduct bio-electrical signals, but too much salt causes water to be sucked out of our cells, as it equalizes the salinity across the water content of our body.

Just to add on what other people have said, route of administration matters. Saline to treat dehydration is given intravenously not orally which is an important difference.

I read thing a few years back, people at sea on a liferaft survived by seawater enemas.

Your body requires salts to function. Without salts your neurons would cease to signal each other and the cells in your body would stop working.

If you just drunk ultra pure water you would die as it would flush the salts from your body.

The difference between a medicine and a poison is dosage. While sea water is saline and hydration fluids is saline, hydration fluids are 0.9% saline sea water sits around 35%.

There are recommended daily intakes of everything things your body needs including water, sodium, chloride, potassium, glucose, protein, vitamins etc. If you become dehydrated you have been losing mainly sodium and water, therefore both need replacing. Sea water has too much salt, too much salt damages your kidneys among other things.

When enough salt (sodium) is given, other intravenous fluids such as dextrose fluid (sugar water) is given, the limits for sugar are much higher than salt so it doesn't matter so much.

Additionally, giving pure water intravenously is not a good idea because if it doesn't have enough electrolytes or molecules like sodium or dextrose it can have an osmotic effect i.e. Water will move into the cells rather than stay in the veins where it is needed. This can swell and damage organs. Drinking pure water can cause this too but its less likely.

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False premise. Drinking salt water doesn't necessarily dehydrate the body. It's just that to avoid that particular effect, you have to drink much smaller quantities at a time, more consistently over long periods of time.