Honestly, the ocean is a harsh client. Salt spray, high humidity, strong UV, wind-driven sand, and condensation cycles work together to rust ferrous metals, fade coatings, and rot ordinary wood. If I had to choose one material for an urban bench near the ocean, I would put marine-grade 316 stainless steel at the top of the list for the frame and hardware. It contains molybdenum, which gives it far better resistance to chloride pitting than standard 304 stainless. 304 can survive inland, but near saltwater it often develops tea staining or small rust spots. 316 costs more, but it is the closest thing to a set-and-forget metal bench.
For most city budgets, powder-coated aluminum is the best all-round body material. Aluminum itself does not rust, and a quality marine-grade powder coat protects the surface from salt air and UV fading. Look for coatings that meet AAMA 2604 or 2605 standards. You still want to rinse it occasionally, especially if the bench is close enough to receive direct salt spray, but it will hold up far better than painted steel.
If you want a warmer, wood-like look, I would skip standard timber and use recycled plastic lumber, often called HDPE. It does not absorb salt moisture, it will not rot, splinter, or require sealing, and it is available in colors that mimic wood. The trade-offs are weight and heat retention in dark colors. Choose lighter colors for sunny coastal locations.
For natural wood, the realistic options are dense tropical hardwoods such as Ipe or Teak, or thermally modified wood such as thermally modified ash. They can survive marine conditions if you use stainless fasteners and accept that the surface will weather to gray. They are beautiful, but they need periodic cleaning and oiling if you want to maintain the original color.
Concrete is excellent for legs, base elements, or monolithic bench forms, but only if it is specified properly. Use low-permeability concrete, corrosion-inhibiting admixtures, and coated or stainless reinforcement. Standard steel rebar too close to the surface can corrode and cause spalling in a marine environment.
Avoid untreated carbon steel, cast iron without a marine paint system, standard powder-coated steel, cheap zinc-plated fasteners, pine, and most softwoods. These are the materials that make oceanfront street furniture look tired within a few years.
My typical specification would be: 316 stainless steel frame or powder-coated aluminum frame, HDPE or thermally modified wood slats, and 316 stainless fasteners throughout. That combination balances long-term durability, maintenance, comfort, and appearance.