Living near the ocean all my life for the most part, even aluminum is still going to corrode, oxidize & pit if left out and exposed. Whatever you decide for frame material, if you can store it in a controlled AC environment, clean it after rides, that's the only way I know of having one last forever. I'll try and snap a few pics of seaside bike racks to get an idea of what leaving different bike frame materials exposed does to them for even a few days, weeks or months. But for the most part, living just over a seaside sand dune, the problem is sea spray & mist. The surf crashes on shore, rocks and the seawater becomes airborne and the wind carries it and anything stationary it lands on. Homes, boats, bikes, cars are higher maintenance, even replacement. In that regard, stainless steel seems to be the choice for boats, maybe even a bike frame ? That rusts too, metal is metal, but SS, that stuff is buffed and polished to a chrome like appearance. But whatever else on your bike that isn't SS, that'll rust. SS isn't cheap either, so that might get beat down in your decision. I don't know if anyone has ever tried it, but zinc plates like what a boat uses when it's docked for prolonged periods. It just doesn't take long for seawater to mess up metal and the fear of the vacation where you show up and the bike is one big rusted and corrosion bubble might even merit a purchasing decision for a bike carrier and taking your bike to and from.

http://www.wagnercompanies.com/Polis...ess_Steel.aspx