Stainless steel is not 'rust proof' it is mostly rust retardant, it has a chromium coating that reduces oxidization but does not stop it completely. The more chromium, the better the rust resistant properties.
If you want (much) better rust resistance than that you are going to be into coatings or active protection such as cathodic protection using a sacrificial material.
Coatings work well as long as there is no mechanical abrasion of the coating, cathodic protection works very well until you run out of sacrificial material.
I was not familiar with these grades, very interesting reading and finding out about them. The most common grades of stainless I have encountered in the marine industry is either 316 (best) or 304 (budget, requires regular cleaning to keep rust free). Both have issues with crevice corrosion and are also very expensive compared to zinc-plated equivalents (which I would never use on a boat since they will effectively disintegrate over a few months). Marine grade stainless often suffers from crevice corrosion and hard to detect failures which is sort of terrifying when you really think about it (things look OK until one day they break without clear visible warning).
For bridges or large-scale industrial applications where you dont care how the metal itself appears, I agree that coatings (especially galvanization) is the best bang for the buck. All of my insights are purely anecdotal though, as a hobbyist...
I worked for a sailmaker in the Netherlands who also did rigging. You really get to appreciate the difference if you see saltwater exposed rigging after a year or two from slightly different steel formulations, one still pristine, the other looking as though someone is pulling a bad joke on your with rust colored paint. Also: maintenance made all the difference, people that would immediately fix a small issue were often able to arrest it without further damage. But ignore it for a while and one thing leads to another.
edit: as to your point regarding crevice corrosion: yes, that is very nasty indeed, especially because a visual inspection will typically turn up nothing out of the order, all it takes is a bit of trapped moisture and some time.
most objects like this I have passed in the water seem to be made of some sort of painted steel. I think this is a good combination of durability vs economical. I suspect your electronics will go long before anything corrodes away to nothing, since in my experience its always the electronics that fail first (even when properly sealed for marine use).
Note that the main issue with any barrier coating (assuming there are no collisions) is going to be due to sea life living on/underneath your object and slowly breaking down the material. A copper based metal (read bronze) could be interesting (but expensive) since it tends to stand up fairly well in marine environments and has biocidal properties that prevent fouling/growth. Also very clear to visually inspect since green means good and red means bad which I always thought was very easy to remember.
edit: re-reading your initial comment it looks like you are describing an autonomous vessel of some sort. I used to work at a company that produced such devices and they were made out of composite plastics (which have their own issues that cause them to break down eventually in the water as well).
I'd look at the nickel alloys, Inconel 625 and Hastelloy C22 would be a good start. They should be orders of magnitude more corrosion resistant than stainless steel and still have respectable strength. Of course they are more expensive and harder to fabricate.
"Stainless steel is not 'rust proof' it is mostly rust retardant, it has a chromium coating that reduces oxidization but does not stop it completely."
Just to be clear ... you can indeed coat things with stainless steel.
However, most stainless steel objects (like screws or bolts or tools, etc.) are stainless throughout - you cannot abrade or scratch into a non-stainless inner core.
This is unlike, for instance, galvanized hardware which is merely steel with a coating over the top. Galvanized items can, indeed, be mechanically altered to reveal non-galvanized material underneath.
As for my grandparents question:
You can, indeed, buy stainless steel beams, rebar[1], etc. They have all the fantastic properties you imagine and are, again, not merely coated like (for instance) galvanized rebar. They are also extremely expensive.
So this isn't about 'objects coated with stainless steel' it is about objects made entirely from stainless steel. Have a look at what a typical all through stainless steel terminal on an ocean going ship looks like after a couple of years. Especially when they are made from lower grades of stainless (such as for instance 304) vs better ones. Not all stainless is made equal when it comes to corrosion resistance.
For bridges or large-scale industrial applications where you dont care how the metal itself appears, I agree that coatings (especially galvanization) is the best bang for the buck. All of my insights are purely anecdotal though, as a hobbyist...