Your body needs a small amount of salt (electrolytes) to operate, and the more you sweat and get dehydrated the more of these salts you lose.

So drinking fluid with an appropriate amount of electrolytes helps hydrate you.

The problem with ocean water is that it has waaaayyyy too much salt and your body reacts to this by trying to get it out of your system via your urine. So you use more water to get it out of your system that you are taking in.

It's a matter of maintaining the appropriate balance.

The human body works best when there is ~9g of salt/liter (based on normal saline used in medicine) which is necessary for electrical signals and chemical reactions that the body needs to keep functioning.

Seawater, by contrast, contains ~35g of salt/liter, more than tripple the concentration of the human body.

This means that drinking seawater intoduces far too much salt, but drinking water with no salt at all can dilute the salt concentration of the body, as some salt is lost via urine and sweat. Either of these extremes can cause problems.

So...you can actually drink sea water (for a limited time) in order to hydrate yourself. The thing is, in quantities high enough to maintain that hydration (as opposed to staving off dehydration), you'd have to drink too much of it over too short a period of time, overloading your kidneys' ability to filter out the excess salt, resulting in hypersalinity. And that's Bad. But, for short periods at the right rate, it is possible.

In the end, it's all about balance: if you go into a situation severely depleted in electrolytes (of which salt is only one option), and drink sea water, it can actually help, up until the balance is restored, and then you start tipping the scales in the other direction.

Homeostasis is key.

The mental model you present in your question is absolute, that salt hydrates or dehydrates. The truth is somewhere in between those extremes.

In this case, your body has a certain salt concentration it likes to maintain, so when you hydrate, it will try and maintain that concentration. If it can't it will either reject some of that water (if there's not enough salt) or require more water (if there's too much salt). A fluid that hydrates optimally contains the exact salt concentration your body prefers.

Electrolytes are a type of salt (the generic name for all ionic compounds in chemistry), but are not table salt (the common vernacular meant by salt). We need some ions, but the right kind in the right balance to function. If you drink very salty water (like the ocean), there is a process called osmosis that will suck water out of you instead, literally dehydrating you.

short answer: Osmosis.

your kidneys rely on osmosis ( an effect that "pulls" water form low concentration mixes to higher ones to equalise the mix strengths) to move waste materials out of the blood and into urine.

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thus, to shift a lot of salt out of the body, you need to loose a lot of water, which dehydrates you.

You CAN drink sea water. Your body would use both the water and salt. But, you just can't drink very much, and not without drinking something else, preferably nu e fresh water, to compensate and help it process the salt.

Like with most things, the poison is in the dose. Even at ypur most dehydrated, salt water from the ocean is too salty, way too salty, and not only will it wreck your kidneys, but without the other electrolytes beside sodium muscles and nerves won't work.