I have read where carbon steel is recommended over stainless for the bolts in the coupling due to the fact stainless will fatigue (and then fail) at a greater rate than carbon steel (use high strength US made). That said my coupling also has stainless bolts from PO. I plan to switch out to a high strength carbon bolt in the future.
I don't know of anyone who would recommend carbon steel over stainless for boat use. Unless you are talking about boats made from the ground up with carbon steel, and have full time work crews aboard to scrape and paint.
304 is ok, 316 is better, I would argue that titanium is best (I am biased as a salesman, but I became a salesman because I believe in the products).
The reality is that strength alone is a poor determiner of which part to buy. It is important, but most rigging is built far stronger than it needs to be to handle the loads imparted by a rig. Almost always the failure point is from corrosion attacking the rigging, and weakening the something that leads to failure. In engineering this is called the Corrosion Allowance, and it is the designed amount of material that can fail before the system does.
304/316 both suffer from crevice corrosion, which is particularly dangerous in areas like chain plates that have a portion of the material bedded into a sealed environment. This causes a low oxygen environment, which prevents the chromium from oxidizing and forming a protective cover over the part. Because of the way that stainless (all grades) corrodes though this can start with just a scratch, and slowly dig its way into the metal littlerly rotting it from the inside out.
Mild steel just flakes apart in marine environments. It can be slowed down by painting, and scraping, but it requires a lot of upkeep, and for parts with pad bearing surfaces (the holes in a Chainplate) it is almost impossible to keep these areas from rusting.
Honestly the best solution is titanium, yes it is expensive (though likely not as bad as you might think), but it is completely immune to corrosion in ambient marine environments. Strait size for size replacements will also save around 50% the weight of stainless parts, while increasing the strength by about 250%. A designed from scratch titanium part can save even more weight, as much as 80% depending on the part.
I don't want to overstate the risks of stainless steel though, it has been used for years, and is a very corrosive resistant metal. There is nothing wrong with 300 series steel, but there are better alternatives.