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The key feature of antifreeze--as the name implies--is that it doesn't freeze.
When I was a kid, my Dad used to use just plain water in the radiator of his 1967 Oldsmobile Cutlass during the summer. It worked ok. He'd only use antifreeze in winter.
Since salt significantly lowers the freezing temperature of water from 32 to -6F (0 to -21C) [1], can salt water be used as a cheap alternative to antifreeze in winter and below-freezing temperatures?
Uh ... absolutely not. Salt water will cause corrosion with in the engine block. Salt water is an electrolyte, so will pass electricity which will cause electrolysis. Also, it might be able to handle a bit lower/higher temps than straight water, it doesn't work as well as antifreeze. Antifreeze is usually good to -40°F/-40°C, plus it protects your engine from electrolysis and corrosion. I'd use straight water before I'd use saltwater ... not something which is good for your engine.
Terrible idea.
Of course you know you can dissolve things like salt into water. You probably also know there's a limit to how much it can hold - called saturation. Saturation is dependent on temperature.
If you saturate warm water and then let it get cold, the material will precipitate - in salt's case, crystallize on a solid surface. Where will it do this? Where it is the coldest, and since your use-case calls for temperatures below 0C/32F, that will be in the radiator, inside the fine passages.
This is a perfect recipe for clogging the radiator.
But more worrisome, with the salt not in the water, it is more vulnerable to freezing. The colder the weather, the poorer this "antifreeze" performs.
As discussed in Paulster2's answer.
You can protect the engine with zinc plates, and you already are - the radiator has a lot of zinc. The radiator will sacrifice itself to save the engine, so good news there anyway.
Salt-water (sodium chloride) solution was, and often still is, used for weight/ballast inside tractor and equipment tires, as that cheap alternative to antifreeze in winter.
Very nasty stuff and highly corrosive to metals not prepared for it. No way would I want that inside an engine block!
Interestingly current trends are towards using antifreeze and other less corrosive liquids.
Sources: personal experience, and any search for “fluid filled tires”.
Most modern engines have aluminium heads (or other components) for reduced weight, and absolutely require antifreeze in the coolant for it's anti-corrosion properties. Aluminium heads at temperature are prone to corrosion and using tapwater, other than quite temporarily, will allow corrosion to start.
Use antifreeze if you don't want to pay for a new engine (or at least, the head gaskets). Choose a quality one, especially if you have aluminium heads -- corrosion can eventually occur with inferior ones. Coolant only needs to be changed occasionally; perhaps 2 years for green, 5 years for long-life antifreeze; so getting this right is a very minimal cost.
In at least a couple of instances, I have observed once tapwater was been used in an aluminium engine for a more than a month or two, issues tended to arise switching back to antifreeze -- eg. scale in the engine lifted and deposited in the radiator, tending to block it. So even temporary use may have costs.
Salt water, as other posters have said, will enable galvanic corrosion and greatly increase the rate of corrosion over tapwater. Don't even consider it.
In addition to the corrosive nature of salt water antifreeze does another duty--it has a higher boiling point than water.
I have not hear about using salt water to cool combustion engines.
But salt water IS used as coolant in commercial cooling equipment. You could find it in large supermarkets, or in office centers, as it is very convenient, that all equipment in supermarket connected to shared cooling network.
For example, they use large industrial chillers mounted on some place external of building (on roof or on some field near building), and feed salt water via ordinary water pipes to building, where industrial refrigerators and conditioner blocks connected to those pipes.
Some marine engines are cooled directly by the overboard water and some of them tolerate salt water as well.
Other than that, @Harper answer sums it best.
Edit: If I was in a dire need to travel in a sub-freezing conditions (or any other conditions where only e.g. sea water is available, I would pretty much try. Few hours of salty water can't be as bad as 10 years of italian antifreeze.
If the car has any value above the scrap by weight - the cooling system can be flushed from the salt.
I believe the antifreeze also acts as a lubricant to moving parts if I'm thinking right. Water not a lubricant plus adding salt might be bad for that part.
The main thing that came to mind is rust. Salt water will cause this in many metals. So you would need to look into the material. Remember you usually need iron present (for most commonly used alloys) for rust (but not oxidisation, which is a different ball game. So any form of steel or iron and I wouldn't put salt anywhere near it. Probably safer with copper, alumnium, titanium etc.
Plain water is indeed fine in the summer. In the winter your best bet is just antifreeze unless like written above you are sure the metal won't rust. There are other alternatives to antifreeze but they are mostly just forms of alcohol (ehthylene gycol) which is what antifreeze is anyway.
