If I were specifying urban outdoor furniture for a coastal city, I would not look for one miracle material. I would match the material to the exposure, the maintenance budget, and how public the piece is. Salt air, humidity, wind-driven rain, and strong UV are the real test. In my experience, the best-performing choices are marine-grade 316 stainless steel, powder-coated aluminum, UV-stabilized HDPE, good teak or dense tropical hardwood, and well-made fiberglass or composite.
For frames, fixings, and high-risk details, marine-grade 316 stainless steel is the safest bet. It is not cheap, but it holds up far better than 304 stainless in salt-heavy air, especially near the waterfront. I would use 316 for bolts, brackets, leg bases, and any place where a small failure could become a safety issue. It still needs occasional rinsing to remove salt and grime, and welds should be properly passivated. I would avoid mild steel, even galvanized mild steel, unless it is very well coated and easy to inspect. Coastal cities are unforgiving to hidden corrosion.
Powder-coated aluminum is one of the most practical choices for coastal urban outdoor furniture. It is light, does not rust like steel, and can be formed into benches, table frames, bins, and bollards. The key is a good architectural-grade powder coating and careful detailing. Cut edges should be sealed, and stainless or nylon washers should be used where aluminum meets other metals to reduce galvanic corrosion. If the coating is chipped, it should be touched up quickly. For seafront locations, I would choose a light color to reduce heat build-up and a matte or textured finish to hide minor wear.
For seat slats and bench tops, UV-stabilized HDPE is excellent. It does not absorb water, does not rot, and does not need painting or oiling. In a coastal city, that low maintenance matters. It is also comfortable enough for public seating if the slats are properly spaced and edges are rounded. The main drawbacks are that low-grade recycled plastic can fade or become brittle, and dark colors can get hot in direct sun. I would specify a reputable UV-stabilized product, light colors, and stainless fixings. HDPE is especially good for beachfront promenades, parks, and bus stops where cleaning crews do not have time for constant refinishing.
Teak is still a strong option for premium outdoor seating, but only if it is genuine quality teak or a comparable dense hardwood. It naturally contains oils that help it resist moisture, and it can silver gracefully if left untreated. In high-use urban settings, I would use FSC-certified teak, stainless steel fixings, and a design that lets water drain freely. It can be oiled if the client wants to keep the golden color, but that adds maintenance. I would not use untreated softwood, and I would be cautious with lower-grade hardwoods that move, splinter, or need frequent sealing.
Fiberglass and composite can work well for shells, curved seats, and planters, but quality varies a lot. A good UV-resistant gelcoat or coating will hold its color longer; a cheap composite may chalk, fade, or crack. Concrete and stone are durable, but they are heavy, can stain, and may spall in salt air if not properly sealed. I would use them for structural or sheltered pieces rather than as the default for seafront seating.
My default specification for a coastal city would be powder-coated aluminum frames with HDPE or teak slats, plus 316 stainless steel fixings. For high-visibility or high-safety elements, I would upgrade to 316 stainless. I would also design for drainage, wind uplift, easy cleaning, and replaceable parts. Coastal outdoor furniture does not need to be complicated, but it does need to be detailed for salt, sun, and maintenance from day one.