Doug

I keep encountering the idea that the sea is good for you because it is salty, like you are, and in some eye-popping examples, that drinking seawater is good for you.

No it isn't, and no it isn't. The sea is much saltier than you are.

Well, if you're suffering from hyponatremia and desperately need extra salt then yes, drinking seawater might be an Ok idea. But people eating a Western diet typically get way more salt than they need already. Generally, drinking seawater in any serious quantity is bad for you. A mouthful or two won't hurt, but more than that will make you thirsty. Switching your main drink to seawater will make you very thirsty indeed and can kill you if you don't get some form of fresh water.

I thought the idea that you can't drink seawater was deeply culturally engrained. Think of all those stories of desperately thirsty shipwrecked sailors. Remember the Rime of the Ancient Mariner:

Water, water, every where,
And all the boards did shrink;
Water, water, every where,
Nor any drop to drink.

Even if you remember the more modern version that Coleridge didn't write: "Water, water, everywhere, yet not a drop to drink". You need fresh, unsalty water to drink.
(Or equivalent quantity of unsalty water in some other form.)

Humans are indeed salty. But nothing like as salty as the sea. Normal saline - the stuff you use for contact lenses and for replacing fluids intravenously - is 0.9% salt, or 9 g per litre. Seawater is about 3.5% salt, or 35 g per litre. (It does vary, but not by enough to get close to saline.) That is much more salty. Like about four times more salty.

I'm pretty sure fresh tears and sweat are about 0.9% salt (in healthy people), because they're formed from blood (which is about 0.9% salt) and it would take a lot of unnecessary work to pump salt in or out. Sweat and tears do get saltier as they dry, until there's just dry salt left on your skin. So it's only partly dried-out tears and sweat that are as salty as the sea. (Sweat also has a little nitrogenous waste in it. And both have all sorts of other biologically interesting things in.) Seawater stings your eyes because it's too salty, in a similar way to how fresh water stings your eyes because it's not salty, although IME seawater is less bad. (Swimming pool water stings your eyes the worst, partly because it's less salty than your eyes, but mainly because of the disinfectant or - more commonly - byproducts from it, typically chloramines from reaction between the hypochlorite disinfectant and urea and related nitrogenous waste compounds from sweat and piss.)

Anyway, that's getting in to more detail than we need. I'm also not getting in to what particular salts are involved; that detail can matter, but only if the gross osmolarity considerations are satisfied.

Playing in the sea can be lots of fun. There are many things from the sea that are tasty, nutritious, and/or useful. But none of that is because the sea is the same saltiness as the human body. It isn't.

There is way, way more salt in seawater than in people. Your tears are much less salty than the sea, until they dry out. Drinking seawater is a particularly bad idea.