A well-built steel hull ought to last a few lifetimes. "Well built" means plate of a thickness appropriate to the strains it will have to withstand, and seams welded evenly and at the appropriate thickness. Bulkheads and stringers placed appropriately so as to ensure stiffness.
Keep a steel hull painted above and below the waterline, and your worries will be few. There are lots of coatings on the market, and builders these days typically deliver new boats painted with two-part epoxy, which means you're married to the same coating system. If it gets dinged, superficial rust is easily fixed, but then you have to mix up a batch of primer on a day when the weather is just right, then do likewise with the finish coat. It can be tedious, but it is not difficult.
Electrolysis is a hazard to any metal, so the steel boat owner wants to assess the environment where the boat is berthed, and make sure the on board electrical system stays properly isolated.
With those things under responsible management, salt water or fresh shouldn't make much difference. A steel hulled boat is rugged and durable. It can be a great comfort in tough conditions. In terms of maintenance burden and longevity, it meets or beats fiberglass on most days.
Wooden boats are a whole different story. I've owned one (or it owned me), and will not do so again.
"Less judgment than wit is more sail than ballast. Yet it must be confessed that wit give an edge to sense, and recommends it extremely." ~ William Penn