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I suspect that rust is the reason that plastic boats traditionally use foam-filled fiberglass beams/frames instead of metal ones in the bilge.
If you use aluminum, many alloys have no tolerance for salt water--so make sure of what you are getting. Stainless disintegrates in anaerobic water, so it can also be unsuitable unless you keep the bilge dry. Either way, you'd want to prime, coat, galvanize, epoxy coat...protect the hell out of the finished piece before installing it.
Personally I'd rather build it up from fiberglass, or something impervious like Delrin or UHDPE (UHWMPE).
Adequately built, properly protected, and regularly inspected--and any of them will do the job fine. Wouldn't even need stainless, plain steel will work if (again) protected & all. Epoxy-impregnated wood, too.