Let me give you the short answer first: for a waterfront location with salty air, I would choose a marine-grade aluminum bench with a powder-coated finish and 316 stainless steel hardware. If aluminum is not the look you want, a recycled plastic bench is a very close second.
A lot of people assume stainless steel is automatically the best choice for salty air. The truth is that not all stainless is equal. Near the coast you want 316 marine-grade stainless rather than 304. The 316 has molybdenum, which makes it much better at resisting pitting and chloride corrosion. Cheap zinc or chrome-plated fasteners are usually the first thing to fail on a waterfront bench, so if you find a bench with a good frame but cheap bolts, plan on swapping them out before installation.
Marine-grade aluminum is my top pick because aluminum itself does not rust the way steel does. When you add a thick architectural powder coat, you get a tough barrier against salt spray, moisture, and UV damage. That combination holds up extremely well on boardwalks, promenades, marinas, and harbor overlooks. Look for a bench that specifically calls out marine-grade or coastal use. Avoid standard aluminum frames paired with steel hardware, because the contact points can still corrode.
Recycled plastic lumber is my second recommendation, and in many public waterfront projects it is actually the most practical choice. It is made from HDPE, so it does not absorb water, rot, splinter, warp, or delaminate. Salt air has essentially no effect on the material. It never needs painting or sealing, and a simple fresh-water rinse removes salt film. The main downside is that recycled plastic can look less traditional than aluminum or wood, but in a coastal setting that is rarely a dealbreaker. If you choose a recycled plastic bench, make sure the frame and hardware are also rated for coastal exposure. Stainless steel inserts and through-bolted connections will help it last far longer.
If you love the look of wood, teak is the only wood I would seriously consider for a salty air waterfront bench. Teak has natural oils that resist moisture and decay, and it weathers to a handsome silver-gray if left untreated. The catch is cost and upkeep. Teak is expensive, and if you want to keep that golden color you will need to clean it and occasionally apply a suitable treatment. Even with teak, I would insist on 316 stainless steel hardware. Avoid softwoods, untreated hardwoods, and most standard wooden benches near the water unless you have a dedicated maintenance program.
There are several things I would avoid in a waterfront setting. Standard steel with only a wet-coat paint finish usually starts rusting within a season or two. Cast iron can be durable, but only if it is historical and properly maintained. Hollow tube steel can collect salty condensation inside and rust from the inside out. Cheap fasteners, basic zinc bolts, and untreated metal frames are common failure points. If a bench looks good but has no coastal hardware spec, assume it will need upgrades.
The details matter just as much as the material. A good waterfront bench should have an open or slatted design that lets air move and water drain. Salt should not be trapped in joints or under decorative caps. Through-bolted connections are better than self-tapping screws. Light colors such as sand, sea gray, or white are smart because they stay cooler in direct sun and hide salt residue better than dark finishes. If the bench is right on the water, a fresh-water rinse every few weeks will keep salt film from building up and extend the life of the finish.
If I had to choose one specific setup, I would pick a six-foot slatted bench with recycled plastic or marine-grade aluminum slats, a powder-coated aluminum frame, 316 stainless steel bolts, and a sand or light gray finish. That combination is rust-resistant, comfortable, and far less of a maintenance headache than most traditional benches. It is exactly the kind of waterfront bench I would recommend for salty air, whether the location is a public pier, a seaside park, a marina walkway, or a private coastal property.
If your budget is tighter, go with a well-built recycled plastic bench from a commercial brand rather than a cheap metal bench. Near the coast, recycled plastic will almost always outlast a budget steel bench. Whatever you choose, check the hardware specification before buying. That is usually the detail that separates a bench that lasts from one that starts bleeding rust down its legs after the first season.